diff --git a/.github/scripts/changelog-monotonic.sh b/.github/scripts/changelog-monotonic.sh
index 16f681b..904efdf 100755
--- a/.github/scripts/changelog-monotonic.sh
+++ b/.github/scripts/changelog-monotonic.sh
@@ -83,11 +83,12 @@ merge_base="$(git merge-base "$base_ref" HEAD 2>/dev/null || true)"
 # version. Field $2, the same split changelog-armed.sh and release-notes.sh
 # use, so the three cannot disagree about what a section header is.
 # 'Unreleased' fails the shape and is excluded by construction.
-headings() {
+headings_raw() {
   awk '
     /^## / && $2 ~ /^[0-9]+\.[0-9]+\.[0-9]+/ { print $2 }
-  ' | sort -u
+  '
 }
+headings() { headings_raw | sort -u; }
 
 # The changelog may not exist at the merge base at all (the commit that adds
 # it). Nothing to have deleted, so nothing to assert.
@@ -97,6 +98,57 @@ base_file="$(git show "$merge_base:$changelog" 2>/dev/null || true)"
   exit 0
 }
 
+# --- uniqueness on HEAD (the #118 class) -------------------------------------
+# Containment catches a DELETED heading. It cannot catch a DUPLICATED one: the
+# duplicate is head-side SURPLUS, and `comm -23` (base minus head) is blind to
+# extras on the head side — with or without `sort -u`, base {0.8.0} minus head
+# {0.8.0, 0.8.0} is empty. Multiset comparison does not close it either, for
+# the same reason. The assert that does is uniqueness of version headings ON
+# HEAD, kept alongside containment rather than replacing it.
+#
+# This is the shape #118's bad rebase actually produced: two
+# `## 0.8.0 — 2026-07-19` headings with the incoming entry between them. Every
+# other guard stayed green — markers absent, changelog-armed.sh happy (the top
+# section was still right), tests and shellcheck clean — while
+# release-notes.sh re-armed its grab on the second heading and folded post-cut
+# prose into the shipped release body.
+#
+# Nothing legitimate repeats a version heading: the ceremony stamps a NEW
+# version, and 'Unreleased' fails the version shape and never reaches here.
+dupes="$(headings_raw < "$changelog" | sort | uniq -d)"
+if [ -n "$dupes" ]; then
+  {
+    echo "changelog-monotonic: $changelog has DUPLICATE release heading(s):"
+    echo
+    printf '%s\n' "$dupes" | sed 's/^/    ## /'
+    echo
+    cat <<EOF
+  Each version heading must appear exactly once. A repeat splits one release
+  into two same-named sections, and release-notes.sh re-arms its extraction on
+  every matching '## ' line — so the published body for that version absorbs
+  whatever sits between the copies, and an entry stranded there is dropped from
+  the NEXT release's notes as well.
+
+  This is the #118 shape: an entry meant for '## Unreleased' was inserted after
+  a shipped heading, and the heading re-added below it. The fix is one heading,
+  with the entry above it under '## Unreleased':
+
+      ## Unreleased
+
+      ### Fixed
+
+      - **Your entry**
+
+      ## $(printf '%s\n' "$dupes" | head -1) — DATE     <- exactly once
+
+  Quick check on any changelog-touching rebase:
+
+      diff <(git show origin/main:$changelog | grep '^## ') <(grep '^## ' $changelog)
+EOF
+  } >&2
+  exit 1
+fi
+
 base_headings="$(printf '%s\n' "$base_file" | headings)"
 head_headings="$(headings < "$changelog")"
 
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 34f1db0..e33cef1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -103,6 +103,18 @@ which records not just what changed but what each drill run proved.
   `CHANGELOG_MONOTONIC_STRICT=1` and checks out with `fetch-depth: 0` so the
   guard can never quietly stop guarding.
 
+  Review of this PR found the guard's first cut incomplete, and the gap is the
+  shape the incident *actually* had. Containment catches a **deleted** heading;
+  it cannot catch a **duplicated** one, because the duplicate is head-side
+  surplus and `comm -23` (base minus head) is blind to extras on the head side
+  — with or without `sort -u`, and multiset comparison does not close it for
+  the same reason. So the guard now also asserts that version headings are
+  **unique on HEAD**, alongside containment rather than instead of it. Nothing
+  legitimate repeats one: the ceremony stamps a new version, and `Unreleased`
+  fails the version shape. Both trees are pinned in `test/release.sh` — the
+  deletion near-miss and the real duplicate — each with `changelog-armed.sh`
+  asserted green on it, which is the whole reason this script exists.
+
 ## 0.8.0 — 2026-07-19
 
 ### Added
diff --git a/test/release.sh b/test/release.sh
index c68b734..acb0f46 100644
--- a/test/release.sh
+++ b/test/release.sh
@@ -335,6 +335,27 @@ check "monotonic: ...and why nothing else says so (git merges it cleanly)" 1 "gi
 check "monotonic: ...on a tree changelog-armed.sh calls FINE (the #122 gap)" 0 "agrees" \
   bash "$ARMED" "$G/CHANGELOG.md" "$ROOT/VERSION"
 
+# --- the #118 incident as it ACTUALLY happened: a DUPLICATED heading -------
+# The deletion case above is the near-miss. What the bad rebase really produced
+# was two '## 0.8.0 — 2026-07-19' headings with the incoming entry stranded
+# between them. Containment cannot see this: the duplicate is head-side
+# SURPLUS, and `comm -23` (base minus head) is blind to extras on the head side
+# — with or without `sort -u`, and multiset comparison does not close it for
+# the same reason. Uniqueness on HEAD is the assert that does.
+G="$(grepo mono-dup '## Unreleased' '' '## 0.8.0 — 2026-07-19' '' '### Added' '' '- **Shipped prose**')"
+head_changelog "$G" '## Unreleased' '' '## 0.8.0 — 2026-07-19' '' '### Fixed' '' '- **An entry**' '' '## 0.8.0 — 2026-07-19' '' '### Added' '' '- **Shipped prose**'
+check "monotonic: a DUPLICATED release heading fails (the #118 shape)" 1 "DUPLICATE release heading" mono "$G" main
+check "monotonic: ...and names the repeated heading" 1 "## 0.8.0" mono "$G" main
+check "monotonic: ...and says what a repeat does to release-notes extraction" 1 "re-arms its extraction" mono "$G" main
+# Containment alone is green on this exact tree — nothing was deleted. Pinned
+# so a future simplification cannot collapse the two asserts into one.
+# shellcheck disable=SC2016  # $1/$2 are the inner shell's positionals, not ours
+check "monotonic: ...on a tree where NOTHING was deleted (containment is blind)" 1 "" \
+  bash -c 'cd "$1" && bash "$2" main 2>&1 | grep -q "DELETES release heading"' _ "$G" "$MONO"
+# And, as with the deletion case, the other guard calls this tree fine.
+check "monotonic: ...on a tree changelog-armed.sh calls FINE" 0 "agrees" \
+  bash "$ARMED" "$G/CHANGELOG.md" "$ROOT/VERSION"
+
 # --- the release ceremony's stamp: an ADD, never a removal -----------------
 # '## Unreleased' -> '## 0.8.1 — DATE' adds 0.8.1 and removes no X.Y.Z
 # heading, because 'Unreleased' is not one. A false positive here would make