diff --git a/DESIGN/01-storage-format.md b/DESIGN/01-storage-format.md
index 932d623..cfefa31 100644
--- a/DESIGN/01-storage-format.md
+++ b/DESIGN/01-storage-format.md
@@ -81,7 +81,7 @@ Body: lane description / WIP policy / notes.
 
 Body: the card's content — the whole point.
 
-Schema-owned display fields are Lanework's to interpret — **coerce where a sensible reading exists, fall back to the field's default where none does** (settled). A scalar of the wrong YAML type reads as its source text (`title: 2048` displays as "2048", `width: "2"` reads as 2); where no sensible reading exists — a sequence or mapping where a scalar belongs, a non-integer width, an unparseable timestamp — the field falls back to its default: untitled placeholder, width 1, no color, no icon. Coercion is read-side only; the bytes on disk are **preserved verbatim, never rewritten**. One tombstone nuance: any *present* `deleted:` key tombstones the item — an unusable timestamp still deletes, its date merely unknown (the user's intent to delete outranks the broken date). **Duplicate keys: last one wins** (settled — the coercing read; strict YAML would reject the file, so this is a deliberate divergence in the editor's favor): a key appearing twice reads as its last occurrence, earlier occurrences preserved verbatim on disk and invisible. An app write of a duplicated key rewrites the winning (last) occurrence and removes the earlier ones — the app owns the keys it writes, and leaving a stale twin would resurrect it if the winner were later removed; removing a key removes all its occurrences. **App rewrites preserve comments** (settled): comments on their own lines always survive a rewrite; an inline comment on a rewritten value line is re-spliced after the new value — best-effort, guaranteed for plain scalar lines (the realistic case), dropped only in pathological shapes. Fail-fast remains reserved for structure (`schema`, `order`, YAML validity) — and it covers *malformed*, not just missing: an `order` that is present but non-numeric is the same loud malformed-input rejection as a missing one. **Readable-but-uneditable shapes load; writes to them refuse** (settled): frontmatter the surgical editor can't key by spans — a whole-frontmatter flow mapping, non-scalar keys — reads and renders normally, and any app write to that file fails loudly through the per-file write-failure banner (02-architecture.md ▸ Write-failure surfacing) naming the shape, never a silent corruption and never a load rejection.
+Schema-owned display fields are Lanework's to interpret — **coerce where a sensible reading exists, fall back to the field's default where none does** (settled). A scalar of the wrong YAML type reads as its source text (`title: 2048` displays as "2048", `width: "2"` reads as 2); where no sensible reading exists — a sequence or mapping where a scalar belongs, a non-integer width, an unparseable timestamp — the field falls back to its default: untitled placeholder, width 1, no color, no icon. Coercion is read-side only; the bytes on disk are **preserved verbatim, never rewritten**. One tombstone nuance: any *present* `deleted:` key tombstones the item — an unusable timestamp still deletes, its date merely unknown (the user's intent to delete outranks the broken date). **Duplicate keys: last one wins** (settled — the coercing read; strict YAML would reject the file, so this is a deliberate divergence in the editor's favor): a *top-level* key appearing twice reads as its last occurrence, earlier occurrences preserved verbatim on disk and invisible. A duplicate inside a nested mapping value remains a YAML error — the rescue applies where the editor's slip actually happens, the top level. An app write of a duplicated key rewrites the winning (last) occurrence and removes the earlier ones — the app owns the keys it writes, and leaving a stale twin would resurrect it if the winner were later removed; removing a key removes all its occurrences. **App rewrites preserve comments** (settled): comments on their own lines always survive a rewrite; an inline comment on a rewritten value line is re-spliced after the new value — best-effort, guaranteed for plain scalar lines (the realistic case), dropped only in pathological shapes. Fail-fast remains reserved for structure (`schema`, `order`, YAML validity) — and it covers *malformed*, not just missing: an `order` that is present but non-numeric is the same loud malformed-input rejection as a missing one. **Readable-but-uneditable shapes load; writes to them refuse** (settled): frontmatter the surgical editor can't key by spans — a whole-frontmatter flow mapping, non-scalar keys — reads and renders normally, and any app write to that file fails loudly through the per-file write-failure banner (02-architecture.md ▸ Write-failure surfacing) naming the shape, never a silent corruption and never a load rejection.
 
 ## Enhanced schema (reserved, out of scope)
 
diff --git a/Kanban/Storage/FrontmatterDocument.swift b/Kanban/Storage/FrontmatterDocument.swift
index 9936530..e580484 100644
--- a/Kanban/Storage/FrontmatterDocument.swift
+++ b/Kanban/Storage/FrontmatterDocument.swift
@@ -50,6 +50,17 @@ public struct FrontmatterDocument: Sendable, Equatable {
 
     // MARK: - Parsing
 
+    /// Reads a file. YAML is composed to validate and interpret it; the text itself is retained
+    /// verbatim, so the parse only decides how the document *reads*, never how it is written.
+    ///
+    /// **Duplicate top-level keys: last one wins** (01-storage-format.md § Frontmatter) — a
+    /// deliberate divergence from strict YAML, which rejects the file outright. The key's last
+    /// occurrence supplies the value, earlier twins are invisible to every read, and each
+    /// occurrence still keeps its own span, so the file round-trips byte-identically. `keys` and
+    /// `fields` present that effective view: one entry per key, at the position of the occurrence
+    /// that won (`a`, `b`, `a` reads as `b`, `a`) — which is also the order the file itself ends
+    /// up in once `set` collapses the twins. A duplicate *nested* inside a value's own mapping is
+    /// out of scope: it stays `unparseableYAML`, as does everything else libYAML rejects.
     public static func parse(_ text: String) throws(FrontmatterError) -> FrontmatterDocument {
         let lines = Self.lines(of: text)
         guard let first = lines.first, Self.isDelimiter(first) else { throw .missingOpeningDelimiter }
@@ -58,14 +69,7 @@ public struct FrontmatterDocument: Sendable, Equatable {
         }
 
         let yamlLines = Array(lines[1 ..< closingIndex])
-        let node: Node?
-        do {
-            node = try Yams.compose(yaml: yamlLines.joined())
-        } catch let error as YamlError {
-            throw .unparseableYAML(message: "\(error)", line: Self.fileLine(of: error))
-        } catch {
-            throw .unparseableYAML(message: "\(error)", line: nil)
-        }
+        let (node, placeholders) = try Self.composeToleratingDuplicateKeys(yamlLines)
 
         let mapping: Node.Mapping
         switch node {
@@ -81,27 +85,86 @@ public struct FrontmatterDocument: Sendable, Equatable {
             }
         }
 
-        let keys = mapping.map { $0.key.string }
-        let values = zip(keys, mapping).compactMap { key, pair -> KeyedValue? in
-            key.map { KeyedValue(key: $0, value: YAMLValue(pair.value)) }
-        }
+        let keys = mapping.map { pair in pair.key.string.map { placeholders[$0] ?? $0 } }
 
         return FrontmatterDocument(
             openingDelimiter: first,
             closingDelimiter: lines[closingIndex],
             spans: Self.makeSpans(lines: yamlLines, keys: keys),
-            values: values,
+            values: Self.lastWinsValues(keys: keys, mapping: mapping),
             body: lines[(closingIndex + 1)...].joined()
         )
     }
 
+    /// Composes the YAML block, tolerating repeated top-level keys.
+    ///
+    /// libYAML refuses a mapping with a duplicated key, but its error is informative enough to
+    /// disarm: it names the duplicated keys and marks the *first* occurrence of one of them. That
+    /// occurrence's key is renamed to a placeholder and the block re-composed, until nothing is
+    /// duplicated. Renaming leaves the line count — and therefore every subsequent mark, and the
+    /// whole span layout — identical to the original, so the composed mapping still lines up with
+    /// the file's lines one for one; the returned table maps each placeholder back to the key it
+    /// stands for. Anything that is not a duplicate at column 0 (a nested duplicate included) is
+    /// reported as `unparseableYAML`, unchanged.
+    private static func composeToleratingDuplicateKeys(
+        _ lines: [String]
+    ) throws(FrontmatterError) -> (node: Node?, placeholders: [String: String]) {
+        var work = lines
+        var placeholders: [String: String] = [:]
+        let prefix = Self.placeholderPrefix(avoiding: lines.joined())
+
+        while true {
+            do {
+                return (try Yams.compose(yaml: work.joined()), placeholders)
+            } catch let error as YamlError {
+                guard case let .duplicatedKeysInMapping(duplicates, context) = error else {
+                    throw FrontmatterError.unparseableYAML(message: "\(error)", line: Self.fileLine(of: error))
+                }
+                // Marks are 1-based and count only the YAML block, which the rename never resizes.
+                let index = context.mark.line - 1
+                guard placeholders.count < lines.count, work.indices.contains(index),
+                      let key = duplicates.first(where: { Self.lineStartsKey(work[index], key: $0) })
+                else {
+                    throw FrontmatterError.unparseableYAML(message: "\(error)", line: Self.fileLine(of: error))
+                }
+                let placeholder = prefix + String(placeholders.count)
+                let line = work[index]
+                placeholders[placeholder] = key
+                work[index] = placeholder + String(line.dropFirst(Self.keyText(of: line, key: key).count))
+            } catch {
+                throw FrontmatterError.unparseableYAML(message: "\(error)", line: nil)
+            }
+        }
+    }
+
+    /// A key stem that appears nowhere in the file, so a renamed duplicate can never collide with
+    /// a key the author actually wrote.
+    private static func placeholderPrefix(avoiding text: String) -> String {
+        var prefix = "__lanework_duplicate_"
+        while text.contains(prefix) { prefix += "_" }
+        return prefix
+    }
+
+    /// One entry per distinct key, carrying the *last* occurrence's value and sitting where that
+    /// occurrence sits — the last-wins reading of a file with repeated keys.
+    private static func lastWinsValues(keys: [String?], mapping: Node.Mapping) -> [KeyedValue] {
+        var values: [KeyedValue] = []
+        for (key, pair) in zip(keys, mapping) {
+            guard let key else { continue }
+            values.removeAll { $0.key == key }
+            values.append(KeyedValue(key: key, value: YAMLValue(pair.value)))
+        }
+        return values
+    }
+
     public func serialized() -> String {
         openingDelimiter + spans.map(\.text).joined() + closingDelimiter + body
     }
 
     // MARK: - Generic access
 
-    /// Top-level keys in document order.
+    /// Top-level keys in document order — the effective, last-wins view: a key written twice
+    /// appears once, where its winning (last) occurrence sits (see `parse`).
     public var keys: [String] { values.map(\.key) }
 
     public var fields: [FrontmatterField] {
@@ -122,18 +185,29 @@ public struct FrontmatterDocument: Sendable, Equatable {
         values.first { $0.key == key }?.value
     }
 
-    /// The key's value text exactly as written, minus the `key:` header and surrounding whitespace.
+    /// The key's value text exactly as written, minus the `key:` header and surrounding
+    /// whitespace. Read from the winning (last) occurrence, like every other read.
     public func rawValue(for key: String) -> String? {
-        spans.first { $0.key == key }.map { Self.valueText(of: $0.value, key: key) }
+        spans.last { $0.key == key }.map { Self.valueText(of: $0.value, key: key) }
     }
 
     // MARK: - Surgical edits
 
     /// Rewrites only this key's value lines. A new key is appended before the closing delimiter.
+    ///
+    /// Two rules from 01-storage-format.md § Frontmatter land here:
+    ///
+    /// - **Duplicates collapse.** The winning (last) occurrence is rewritten and every earlier
+    ///   twin is deleted in the same call — the app owns the keys it writes, and a stale twin left
+    ///   behind would resurrect itself the moment the winner were removed.
+    /// - **Comments survive.** Comments on their own lines are never touched, and an inline
+    ///   comment trailing the rewritten value line is re-spliced after the new value at its
+    ///   original distance (`order: 3072  # keep at top` → `order: 4096  # keep at top`). That
+    ///   re-splice is best-effort — see `inlineComment(of:keyText:)`.
     public mutating func set(_ key: String, to value: FrontmatterValue) {
-        if let index = spans.firstIndex(where: { $0.key == key }) {
-            let keyText = Self.keyText(of: spans[index].value, key: key)
-            spans[index].value = "\(keyText): \(value.yamlText)\n"
+        if let index = spans.lastIndex(where: { $0.key == key }) {
+            spans[index].value = Self.rewritten(spans[index].value, key: key, to: value)
+            for twin in (0 ..< index).reversed() where spans[twin].key == key { clearSpan(at: twin) }
         } else {
             spans.append(Span(key: key, value: "\(FrontmatterValue.emitScalar(key)): \(value.yamlText)\n", tail: ""))
         }
@@ -146,19 +220,23 @@ public struct FrontmatterDocument: Sendable, Equatable {
         }
     }
 
-    /// Deletes the key's value lines. Trailing comments and blank lines survive — they read as
+    /// Deletes the key's value lines — *every* occurrence of it, so a removed key cannot be
+    /// resurrected by an earlier twin. Trailing comments and blank lines survive: they read as
     /// belonging to whatever follows, and losing user text is worse than an orphaned comment.
     public mutating func remove(_ key: String) {
-        if let index = spans.firstIndex(where: { $0.key == key }) {
-            if spans[index].tail.isEmpty {
-                spans.remove(at: index)
-            } else {
-                spans[index] = Span(key: nil, value: "", tail: spans[index].tail)
-            }
-        }
+        for index in spans.indices.reversed() where spans[index].key == key { clearSpan(at: index) }
         values.removeAll { $0.key == key }
     }
 
+    /// Drops one span's value lines, keeping its trailing comments and blank lines.
+    private mutating func clearSpan(at index: Int) {
+        if spans[index].tail.isEmpty {
+            spans.remove(at: index)
+        } else {
+            spans[index] = Span(key: nil, value: "", tail: spans[index].tail)
+        }
+    }
+
     // MARK: - Line splitting
 
     private static func lines(of text: String) -> [String] {
@@ -176,7 +254,8 @@ public struct FrontmatterDocument: Sendable, Equatable {
 
     /// Splits the YAML block into per-key spans. A line only starts a span when it opens the key
     /// the parser expects next, so an unindented value continuation (multi-line flow collections)
-    /// can never be mistaken for a key.
+    /// can never be mistaken for a key. `keys` lists every top-level occurrence, duplicates
+    /// included, so each twin gets its own span and the block still serializes back verbatim.
     private static func makeSpans(lines: [String], keys: [String?]) -> [Span] {
         var spans: [Span] = []
         var current: Span?
@@ -235,6 +314,118 @@ public struct FrontmatterDocument: Sendable, Equatable {
         keyCandidates(key).first { spanValue.hasPrefix($0 + ":") } ?? FrontmatterValue.emitScalar(key)
     }
 
+    // MARK: - Rewriting a value line
+
+    /// The replacement text for a span's value lines: `key: newValue`, plus whatever inline
+    /// comment the old lines carried, ending the way they ended (a CRLF file stays CRLF).
+    private static func rewritten(_ spanValue: String, key: String, to value: FrontmatterValue) -> String {
+        let keyText = Self.keyText(of: spanValue, key: key)
+        let (comment, terminator) = Self.inlineComment(of: spanValue, keyText: keyText)
+        return "\(keyText): \(value.yamlText)\(comment)\(terminator)"
+    }
+
+    /// The trailing inline comment on a span, its leading whitespace included so it re-splices at
+    /// the distance the author left, along with the line's own terminator.
+    ///
+    /// **Best-effort by design** (01-storage-format.md § Frontmatter). Guaranteed for the
+    /// realistic case — a value written on one line as a plain, single-quoted, or double-quoted
+    /// scalar — where YAML's own rule decides: `#` opens a comment only when preceded by
+    /// whitespace and not inside a quoted scalar, so the `#` in `title: "a # b"` is value text and
+    /// survives as part of the old value rather than being re-spliced. Pathological shapes lose
+    /// the comment instead of making the editor guess: a value spanning several lines (block
+    /// scalar, multi-line flow collection) is not scanned at all, and neither is a single-line
+    /// flow collection whose quotes do not balance.
+    private static func inlineComment(of spanValue: String, keyText: String) -> (comment: String, terminator: String) {
+        // A CRLF pair is one `Character`, so both endings have to be named explicitly.
+        var characters = Array(spanValue)
+        var terminator = "\n"
+        if characters.last == "\r\n" {
+            characters.removeLast()
+            terminator = "\r\n"
+        } else if characters.last == "\n" {
+            characters.removeLast()
+        }
+        // Only a value written on a single line is scanned; anything taller keeps no comment.
+        guard !characters.contains(where: { $0 == "\n" || $0 == "\r\n" }) else { return ("", terminator) }
+        let header = Array(keyText + ":")
+        guard characters.starts(with: header) else { return ("", terminator) }
+
+        var valueStart = header.count
+        while valueStart < characters.count, isSpaceOrTab(characters[valueStart]) { valueStart += 1 }
+        guard let hash = commentIndex(in: characters, valueStart: valueStart) else { return ("", terminator) }
+
+        var start = hash
+        while start > 0, isSpaceOrTab(characters[start - 1]) { start -= 1 }
+        return (String(characters[start...]), terminator)
+    }
+
+    /// Where this line's comment begins, or nil if it has none. The value's opening character
+    /// says how much of the line is off limits: a quoted scalar is skipped whole, a flow
+    /// collection is scanned with its quotes tracked, and a plain scalar cannot contain ` #` at
+    /// all — that sequence is exactly what ends it.
+    private static func commentIndex(in characters: [Character], valueStart: Int) -> Int? {
+        guard valueStart < characters.count else { return nil }
+        var index = valueStart
+        switch characters[valueStart] {
+        case "\"", "'":
+            guard let end = endOfQuoted(characters, from: valueStart) else { return nil }
+            index = end
+        case "[", "{":
+            return flowCommentIndex(characters, from: valueStart)
+        default:
+            break
+        }
+        while index < characters.count {
+            if characters[index] == "#", index > 0, isSpaceOrTab(characters[index - 1]) { return index }
+            index += 1
+        }
+        return nil
+    }
+
+    /// One past the closing quote of the quoted scalar starting at `start`, or nil if it does not
+    /// close on this line. `\"` escapes inside double quotes, `''` inside single ones.
+    private static func endOfQuoted(_ characters: [Character], from start: Int) -> Int? {
+        let quote = characters[start]
+        var index = start + 1
+        while index < characters.count {
+            if quote == "\"", characters[index] == "\\" {
+                index += 2
+                continue
+            }
+            if characters[index] == quote {
+                if quote == "'", index + 1 < characters.count, characters[index + 1] == "'" {
+                    index += 2
+                    continue
+                }
+                return index + 1
+            }
+            index += 1
+        }
+        return nil
+    }
+
+    /// A flow collection on one line: a quote here really does open a quoted scalar, so a `#`
+    /// inside one is value text.
+    private static func flowCommentIndex(_ characters: [Character], from start: Int) -> Int? {
+        var index = start
+        while index < characters.count {
+            switch characters[index] {
+            case "\"", "'":
+                guard let end = endOfQuoted(characters, from: index) else { return nil }
+                index = end
+            case "#" where index > 0 && isSpaceOrTab(characters[index - 1]):
+                return index
+            default:
+                index += 1
+            }
+        }
+        return nil
+    }
+
+    private static func isSpaceOrTab(_ character: Character) -> Bool {
+        character == " " || character == "\t"
+    }
+
     private static func valueText(of spanValue: String, key: String) -> String {
         let header = keyText(of: spanValue, key: key) + ":"
         guard spanValue.hasPrefix(header) else {
diff --git a/KanbanTests/FrontmatterTests.swift b/KanbanTests/FrontmatterTests.swift
index 395a39c..a985b96 100644
--- a/KanbanTests/FrontmatterTests.swift
+++ b/KanbanTests/FrontmatterTests.swift
@@ -69,6 +69,19 @@ private enum Fixture {
         ---
         body
         """
+
+    /// A hand-edited file that says `title` twice — strict YAML would reject it; the editor reads
+    /// the last one and keeps both on disk (01-storage-format.md § Frontmatter).
+    static let duplicated = """
+        ---
+        schema: 1
+        title: First
+        order: 1024
+        title: Second
+        ---
+        body
+
+        """
 }
 
 private func parseError(_ text: String) -> FrontmatterError? {
@@ -122,6 +135,11 @@ struct FrontmatterRoundTripTests {
         "---\nschema: 1\n---\nbody without newline",
         // CRLF throughout
         "---\r\nschema: 1\r\n---\r\nbody\r\n",
+        // duplicate top-level keys — every occurrence keeps its own span
+        Fixture.duplicated,
+        "---\ntitle: a\ntitle: b\ntitle: c\n---\nbody\n",
+        // a duplicate whose earlier twin owns a comment and a block scalar
+        "---\nnotes: |\n  one\n# about notes\nnotes: two\n---\nbody\n",
     ])
     func serializingAnUntouchedDocumentIsByteIdentical(text: String) throws {
         let document = try FrontmatterDocument.parse(text)
@@ -165,10 +183,13 @@ struct FrontmatterMalformedTests {
         "---\nfoo: [1, 2\n---\nbody\n",
         "---\nschema: 1\n\tindented-with-tab: 2\n---\nbody\n",
         "---\nschema: 1\n  bad: indent\n---\nbody\n",
-        "---\ndupe: 1\ndupe: 2\n---\nbody\n",
         "---\n\"unterminated: 1\n---\nbody\n",
         "---\nfoo: {a: 1\n---\nbody\n",
         "---\n*undefined-alias\n---\nbody\n",
+        // last-wins rescues duplicates at the top level only — nested ones stay a hard failure
+        "---\nouter:\n  a: 1\n  a: 2\n---\nbody\n",
+        "---\nflow: {a: 1, a: 2}\n---\nbody\n",
+        "---\ndupe: 1\ndupe: 2\nouter:\n  a: 1\n  a: 2\n---\nbody\n",
     ])
     func unparseableYAML(text: String) {
         guard case .unparseableYAML = parseError(text) else {
@@ -215,6 +236,155 @@ struct FrontmatterMalformedTests {
             #expect(parseError(text) == nil)
         }
     }
+
+    /// Strict YAML rejects a repeated mapping key; the editor deliberately does not.
+    @Test func duplicateTopLevelKeysAreNotAParseError() throws {
+        for text in [
+            Fixture.duplicated,
+            "---\ndupe: 1\ndupe: 2\n---\nbody\n",
+            "---\ntitle: a\ntitle: b\ntitle: c\n---\nbody\n",
+        ] {
+            #expect(parseError(text) == nil)
+        }
+    }
+}
+
+// MARK: - Duplicate keys (last one wins)
+
+/// 01-storage-format.md § Frontmatter: a key written twice reads as its last occurrence, the
+/// earlier ones preserved verbatim on disk and invisible; an app write of that key rewrites the
+/// winner and removes the twins; a removal removes them all.
+struct FrontmatterDuplicateKeyTests {
+    @Test func theLastOccurrenceIsTheOneThatReads() throws {
+        let document = try FrontmatterDocument.parse(Fixture.duplicated)
+        #expect(document.title == .valid("Second"))
+        #expect(document.value(for: "title") == .string("Second"))
+        #expect(document.rawValue(for: "title") == "Second")
+        #expect(document.contains("title"))
+    }
+
+    @Test func threeOccurrencesStillReadAsTheLast() throws {
+        let document = try FrontmatterDocument.parse("---\ntitle: a\ntitle: b\ntitle: c\n---\nbody\n")
+        #expect(document.title == .valid("c"))
+        #expect(document.keys == ["title"])
+    }
+
+    /// The effective view has one entry per key, sitting where the winner sits — the order the
+    /// file itself takes once `set` collapses the twins.
+    @Test func iterationPresentsTheEffectiveView() throws {
+        let document = try FrontmatterDocument.parse(Fixture.duplicated)
+        #expect(document.keys == ["schema", "order", "title"])
+        #expect(document.fields.map(\.key) == ["schema", "order", "title"])
+        #expect(document.fields.last?.rawValue == "Second")
+    }
+
+    @Test func bothOccurrencesSurviveAnUntouchedRoundTrip() throws {
+        let document = try FrontmatterDocument.parse(Fixture.duplicated)
+        let output = document.serialized()
+        #expect(output == Fixture.duplicated)
+        #expect(output.contains("title: First"))
+        #expect(output.contains("title: Second"))
+    }
+
+    @Test func settingRewritesTheWinnerAndDeletesTheTwins() throws {
+        var document = try FrontmatterDocument.parse(Fixture.duplicated)
+        document.set("title", to: .string("Third"))
+        #expect(document.serialized() == "---\nschema: 1\norder: 1024\ntitle: Third\n---\nbody\n")
+        #expect(document.title == .valid("Third"))
+    }
+
+    @Test func settingCollapsesThreeOccurrencesToOne() throws {
+        var document = try FrontmatterDocument.parse("---\ntitle: a\ntitle: b\ntitle: c\n---\nbody\n")
+        document.set("title", to: .string("d"))
+        #expect(document.serialized() == "---\ntitle: d\n---\nbody\n")
+    }
+
+    /// A stale twin left behind would resurrect itself the moment the winner were removed.
+    @Test func removingTakesEveryOccurrence() throws {
+        var document = try FrontmatterDocument.parse(Fixture.duplicated)
+        document.remove("title")
+        #expect(document.serialized() == "---\nschema: 1\norder: 1024\n---\nbody\n")
+        #expect(document.title == .missing)
+        #expect(!document.contains("title"))
+        #expect(document.keys == ["schema", "order"])
+    }
+
+    @Test func removingTakesAllThreeOccurrences() throws {
+        var document = try FrontmatterDocument.parse("---\nschema: 1\ntitle: a\ntitle: b\ntitle: c\n---\nbody\n")
+        document.remove("title")
+        #expect(document.serialized() == "---\nschema: 1\n---\nbody\n")
+    }
+
+    /// Own-line comments outlive the twin they were written above (01-storage-format.md).
+    @Test func aCommentAboveTheFirstOccurrenceSurvivesTheCollapse() throws {
+        let text = "---\n# the original title\ntitle: First\ntitle: Second\n---\nbody\n"
+        var document = try FrontmatterDocument.parse(text)
+        #expect(document.serialized() == text)
+
+        document.set("title", to: .string("Third"))
+        #expect(document.serialized() == "---\n# the original title\ntitle: Third\n---\nbody\n")
+    }
+
+    /// Same rule when the comment trails the earlier twin rather than heading it.
+    @Test func aCommentBelowTheFirstOccurrenceSurvivesTheCollapse() throws {
+        var document = try FrontmatterDocument.parse("---\ntitle: First\n# still relevant\ntitle: Second\n---\nbody\n")
+        document.remove("title")
+        #expect(document.serialized() == "---\n# still relevant\n---\nbody\n")
+    }
+
+    @Test func twoDifferentKeysMayEachBeDuplicated() throws {
+        let text = "---\na: 1\nb: 1\na: 2\nb: 2\n---\nbody\n"
+        var document = try FrontmatterDocument.parse(text)
+        #expect(document.serialized() == text)
+        #expect(document.value(for: "a") == .int(2))
+        #expect(document.value(for: "b") == .int(2))
+        #expect(document.keys == ["a", "b"])
+
+        document.set("a", to: .int(3))
+        #expect(document.serialized() == "---\nb: 1\na: 3\nb: 2\n---\nbody\n")
+    }
+
+    /// A duplicated twin whose value is a block scalar still spans its own lines.
+    @Test func aDuplicateWithAMultiLineTwinCollapsesCleanly() throws {
+        let text = "---\nnotes: |\n  one\n# about notes\nnotes: two\n---\nbody\n"
+        var document = try FrontmatterDocument.parse(text)
+        #expect(document.serialized() == text)
+        #expect(document.value(for: "notes") == .string("two"))
+
+        document.set("notes", to: .string("three"))
+        #expect(document.serialized() == "---\n# about notes\nnotes: three\n---\nbody\n")
+    }
+
+    /// The twins need not be spelled alike — YAML reads `"title"` and `title` as the same key,
+    /// and each occurrence is rewritten (or dropped) in its own spelling.
+    @Test func twinsSpelledDifferentlyAreStillTheSameKey() throws {
+        let text = "---\n\"title\": First\ntitle: Second\n---\nbody\n"
+        var document = try FrontmatterDocument.parse(text)
+        #expect(document.serialized() == text)
+        #expect(document.title == .valid("Second"))
+
+        document.set("title", to: .string("Third"))
+        #expect(document.serialized() == "---\ntitle: Third\n---\nbody\n")
+    }
+
+    @Test func duplicatesInACRLFFileRoundTripAndCollapse() throws {
+        let text = "---\r\ntitle: First\r\norder: 1\r\ntitle: Second\r\n---\r\nbody\r\n"
+        var document = try FrontmatterDocument.parse(text)
+        #expect(document.serialized() == text)
+        #expect(document.title == .valid("Second"))
+
+        document.set("title", to: .string("Third"))
+        #expect(document.serialized() == "---\r\norder: 1\r\ntitle: Third\r\n---\r\nbody\r\n")
+    }
+
+    @Test func duplicatesSurviveReparsing() throws {
+        var document = try FrontmatterDocument.parse(Fixture.duplicated)
+        document.set("order", to: .double(2048))
+        let reparsed = try FrontmatterDocument.parse(document.serialized())
+        #expect(reparsed.serialized() == document.serialized())
+        #expect(reparsed.title == .valid("Second"))
+        #expect(reparsed.order == .valid(2048))
+    }
 }
 
 // MARK: - Generic access
@@ -436,7 +606,7 @@ struct FrontmatterEditTests {
         var document = try FrontmatterDocument.parse(text)
         document.set("project", to: .string("kanban"))
         #expect(document.serialized()
-            == "---\nschema: 1\nproject: kanban\n\n# a note\norder: 5\n---\nbody\n")
+            == "---\nschema: 1\nproject: kanban       # agent overlay\n\n# a note\norder: 5\n---\nbody\n")
     }
 
     @Test func appendingANewKeyGoesBeforeTheClosingDelimiter() throws {
@@ -538,6 +708,112 @@ struct FrontmatterEditTests {
     }
 }
 
+// MARK: - Inline comments on rewritten lines
+
+/// 01-storage-format.md § Frontmatter, "App rewrites preserve comments": an inline comment on a
+/// rewritten value line is re-spliced after the new value — best-effort, guaranteed for plain
+/// single-line scalars, allowed to drop in pathological shapes.
+struct FrontmatterInlineCommentTests {
+    private func edited(_ frontmatter: String, _ key: String, _ value: FrontmatterValue) throws -> String {
+        var document = try FrontmatterDocument.parse("---\n\(frontmatter)\n---\nbody\n")
+        document.set(key, to: value)
+        return String(document.serialized().dropFirst("---\n".count).dropLast("\n---\nbody\n".count))
+    }
+
+    @Test func aPlainScalarKeepsItsCommentAndItsSpacing() throws {
+        #expect(try edited("order: 3072  # keep at top", "order", .double(4096))
+            == "order: 4096  # keep at top")
+        #expect(try edited("order: 3072 # tight", "order", .double(4096)) == "order: 4096 # tight")
+        #expect(try edited("order: 3072\t# tabbed", "order", .double(4096)) == "order: 4096\t# tabbed")
+    }
+
+    @Test func aValueLineWithoutACommentIsRewrittenAsBefore() throws {
+        #expect(try edited("order: 3072", "order", .double(4096)) == "order: 4096")
+        #expect(try edited("title: My Board", "title", .string("Renamed")) == "title: Renamed")
+    }
+
+    /// A `#` inside a quoted scalar is value text, not a comment — it must not be re-spliced.
+    @Test func aHashInsideAQuotedScalarIsNotAComment() throws {
+        #expect(try edited("title: \"hash # inside\"", "title", .string("Renamed")) == "title: Renamed")
+        #expect(try edited("title: 'hash # inside'", "title", .string("Renamed")) == "title: Renamed")
+        #expect(try edited("background: \"#ff8800\"", "background", .string("slate")) == "background: slate")
+    }
+
+    /// …but a real comment *after* such a value still is one.
+    @Test func aCommentAfterAQuotedScalarIsStillSpliced() throws {
+        #expect(try edited("title: \"hash # inside\"  # real note", "title", .string("Renamed"))
+            == "title: Renamed  # real note")
+        #expect(try edited("title: 'it''s # fine'  # real note", "title", .string("Renamed"))
+            == "title: Renamed  # real note")
+    }
+
+    /// An apostrophe mid-scalar does not open a quoted scalar, so the comment after it is found.
+    @Test func aPlainScalarWithAnApostropheKeepsItsComment() throws {
+        #expect(try edited("title: it's fine  # note", "title", .string("Renamed"))
+            == "title: Renamed  # note")
+        #expect(try edited("title: say \"hi\"  # note", "title", .string("Renamed"))
+            == "title: Renamed  # note")
+    }
+
+    @Test func aSingleLineFlowCollectionKeepsItsComment() throws {
+        #expect(try edited("labels: [a, \"b # c\"]  # note", "labels", .raw("[x]"))
+            == "labels: [x]  # note")
+        #expect(try edited("template: {order: 3}  # picker slot", "template", .raw("{order: 7}"))
+            == "template: {order: 7}  # picker slot")
+    }
+
+    @Test func aCommentOnAnEmptyValueIsKept() throws {
+        #expect(try edited("title:  # to be filled in", "title", .string("Named"))
+            == "title: Named  # to be filled in")
+    }
+
+    @Test func theCommentSurvivesSuccessiveSets() throws {
+        var document = try FrontmatterDocument.parse("---\norder: 3072  # keep at top\n---\nbody\n")
+        document.set("order", to: .double(4096))
+        document.set("order", to: .double(8192))
+        #expect(document.serialized() == "---\norder: 8192  # keep at top\n---\nbody\n")
+    }
+
+    @Test func theCommentSurvivesAReparse() throws {
+        var document = try FrontmatterDocument.parse("---\norder: 3072  # keep at top\n---\nbody\n")
+        document.set("order", to: .double(4096))
+        let reparsed = try FrontmatterDocument.parse(document.serialized())
+        #expect(reparsed.order == .valid(4096))
+        #expect(reparsed.serialized() == document.serialized())
+    }
+
+    /// The winning occurrence's own comment is what carries over; a collapsed twin's inline
+    /// comment goes with the twin.
+    @Test func theWinningOccurrencesCommentIsTheOneKept() throws {
+        var document = try FrontmatterDocument.parse("---\norder: 1  # old\norder: 2  # current\n---\nbody\n")
+        document.set("order", to: .double(3))
+        #expect(document.serialized() == "---\norder: 3  # current\n---\nbody\n")
+    }
+
+    /// Pathological shapes are allowed to lose the comment rather than have the editor guess.
+    /// Pinning the chosen behavior: a value spanning several lines is not scanned at all.
+    @Test func aMultiLineValueDropsItsHeaderComment() throws {
+        var document = try FrontmatterDocument.parse("---\nnotes: |  # about the notes\n  line one\n---\nbody\n")
+        document.set("notes", to: .string("flattened"))
+        #expect(document.serialized() == "---\nnotes: flattened\n---\nbody\n")
+
+        var flow = try FrontmatterDocument.parse("---\nflow: {a: 1,\nb: 2}  # spread out\nlast: x\n---\nbody\n")
+        flow.set("flow", to: .raw("{a: 9}"))
+        #expect(flow.serialized() == "---\nflow: {a: 9}\nlast: x\n---\nbody\n")
+    }
+
+    /// Comments on their own lines were always preserved and still are — including the one that
+    /// heads the frontmatter and the one that trails it.
+    @Test func ownLineCommentsAreUntouchedByARewrite() throws {
+        var document = try FrontmatterDocument.parse(Fixture.rich)
+        document.set("title", to: .string("Renamed"))
+        let output = document.serialized()
+        #expect(output.hasPrefix("---\n# board settings, hand-written\nschema: 1\n"))
+        #expect(output.contains("\n# trailing note\n---\n"))
+        #expect(output.contains("project: lanework       # agent overlay\n"))
+    }
+}
+
 // MARK: - Scalar emission
 
 struct FrontmatterEmissionTests {