diff --git a/openspec/changes/agent-codex-prevent-finish-rerun-from-opening-duplic-2026-04-22-23-58/.openspec.yaml b/openspec/changes/agent-codex-prevent-finish-rerun-from-opening-duplic-2026-04-22-23-58/.openspec.yaml
new file mode 100644
index 0000000..25345f4
--- /dev/null
+++ b/openspec/changes/agent-codex-prevent-finish-rerun-from-opening-duplic-2026-04-22-23-58/.openspec.yaml
@@ -0,0 +1,2 @@
+schema: spec-driven
+created: 2026-04-22
diff --git a/openspec/changes/agent-codex-prevent-finish-rerun-from-opening-duplic-2026-04-22-23-58/notes.md b/openspec/changes/agent-codex-prevent-finish-rerun-from-opening-duplic-2026-04-22-23-58/notes.md
new file mode 100644
index 0000000..fa555ff
--- /dev/null
+++ b/openspec/changes/agent-codex-prevent-finish-rerun-from-opening-duplic-2026-04-22-23-58/notes.md
@@ -0,0 +1,20 @@
+# agent-codex-prevent-finish-rerun-from-opening-duplic-2026-04-22-23-58 (minimal / T1)
+
+Branch: `agent/codex/prevent-finish-rerun-from-opening-duplic-2026-04-22-23-58`
+
+`gx branch finish --via-pr --wait-for-merge --cleanup` can reopen a fresh PR when the same source branch head already shipped in an earlier merged PR but the local branch/worktree cleanup was left behind. That turns a cleanup rerun into a new merge-wait loop instead of the bounded cleanup pass the user expected.
+
+Scope:
+- Before PR create/merge, detect whether the current source branch HEAD already matches a merged PR for the same branch/base pair.
+- If that exact head already landed, skip new PR creation and continue straight to local cleanup while preserving merged PR context in the logs.
+- Add a focused finish regression that fails if a rerun opens or merges a duplicate PR for an already-merged head.
+
+Verification:
+- `bash -n scripts/agent-branch-finish.sh templates/scripts/agent-branch-finish.sh`
+- `node --test test/finish.test.js`
+
+## Cleanup
+
+- [ ] Run `gx branch finish --branch agent/codex/prevent-finish-rerun-from-opening-duplic-2026-04-22-23-58 --base main --via-pr --wait-for-merge --cleanup`
+- [ ] Record PR URL + `MERGED` state in the completion handoff.
+- [ ] Confirm sandbox worktree is gone (`git worktree list`, `git branch -a`).
diff --git a/scripts/agent-branch-finish.sh b/scripts/agent-branch-finish.sh
index 3e7e149..456abae 100755
--- a/scripts/agent-branch-finish.sh
+++ b/scripts/agent-branch-finish.sh
@@ -455,6 +455,44 @@ read_pr_state() {
   return 0
 }
 
+read_merged_pr_for_head() {
+  local head_sha="${1:-}"
+  local state_line=""
+  local parsed_state=""
+  local parsed_merged_at=""
+  local parsed_url=""
+
+  if [[ -z "$head_sha" ]]; then
+    return 1
+  fi
+
+  state_line="$("$GH_BIN" pr list \
+    --state merged \
+    --head "$SOURCE_BRANCH" \
+    --base "$BASE_BRANCH" \
+    --json state,mergedAt,url,headRefOid \
+    --jq "map(select(.headRefOid == \"$head_sha\")) | sort_by(.mergedAt // \"\") | reverse | (.[0] // {}) | [(.state // \"\"), (.mergedAt // \"\"), (.url // \"\")] | join(\"\u001f\")" \
+    2>/dev/null || true)"
+  if [[ -z "$state_line" ]]; then
+    return 1
+  fi
+
+  IFS=$'\x1f' read -r parsed_state parsed_merged_at parsed_url <<< "$state_line"
+  if [[ -z "$parsed_state" && -z "$parsed_merged_at" && -z "$parsed_url" ]]; then
+    return 1
+  fi
+  if [[ "$parsed_state" != "MERGED" && -z "$parsed_merged_at" ]]; then
+    return 1
+  fi
+
+  PR_STATE="$parsed_state"
+  PR_MERGED_AT="$parsed_merged_at"
+  if [[ -n "$parsed_url" ]]; then
+    pr_url="$parsed_url"
+  fi
+  return 0
+}
+
 wait_for_pr_merge() {
   local deadline
   deadline=$(( $(date +%s) + WAIT_TIMEOUT_SECONDS ))
@@ -509,11 +547,22 @@ wait_for_pr_merge() {
 }
 
 run_pr_flow() {
+  local source_head_sha=""
+
   if ! command -v "$GH_BIN" >/dev/null 2>&1; then
     echo "[agent-branch-finish] PR fallback requested but GitHub CLI not found: ${GH_BIN}" >&2
     return 1
   fi
 
+  source_head_sha="$(git -C "$repo_root" rev-parse "$SOURCE_BRANCH" 2>/dev/null || true)"
+  if read_merged_pr_for_head "$source_head_sha"; then
+    echo "[agent-branch-finish] Source branch head already landed in a merged PR; skipping new PR creation and continuing cleanup." >&2
+    if [[ -n "$pr_url" ]]; then
+      echo "[agent-branch-finish] Merged PR: ${pr_url}" >&2
+    fi
+    return 0
+  fi
+
   git -C "$source_worktree" push -u origin "$SOURCE_BRANCH"
 
   pr_title="$(git -C "$repo_root" log -1 --pretty=%s "$SOURCE_BRANCH" 2>/dev/null || true)"
diff --git a/templates/scripts/agent-branch-finish.sh b/templates/scripts/agent-branch-finish.sh
index 3e7e149..456abae 100755
--- a/templates/scripts/agent-branch-finish.sh
+++ b/templates/scripts/agent-branch-finish.sh
@@ -455,6 +455,44 @@ read_pr_state() {
   return 0
 }
 
+read_merged_pr_for_head() {
+  local head_sha="${1:-}"
+  local state_line=""
+  local parsed_state=""
+  local parsed_merged_at=""
+  local parsed_url=""
+
+  if [[ -z "$head_sha" ]]; then
+    return 1
+  fi
+
+  state_line="$("$GH_BIN" pr list \
+    --state merged \
+    --head "$SOURCE_BRANCH" \
+    --base "$BASE_BRANCH" \
+    --json state,mergedAt,url,headRefOid \
+    --jq "map(select(.headRefOid == \"$head_sha\")) | sort_by(.mergedAt // \"\") | reverse | (.[0] // {}) | [(.state // \"\"), (.mergedAt // \"\"), (.url // \"\")] | join(\"\u001f\")" \
+    2>/dev/null || true)"
+  if [[ -z "$state_line" ]]; then
+    return 1
+  fi
+
+  IFS=$'\x1f' read -r parsed_state parsed_merged_at parsed_url <<< "$state_line"
+  if [[ -z "$parsed_state" && -z "$parsed_merged_at" && -z "$parsed_url" ]]; then
+    return 1
+  fi
+  if [[ "$parsed_state" != "MERGED" && -z "$parsed_merged_at" ]]; then
+    return 1
+  fi
+
+  PR_STATE="$parsed_state"
+  PR_MERGED_AT="$parsed_merged_at"
+  if [[ -n "$parsed_url" ]]; then
+    pr_url="$parsed_url"
+  fi
+  return 0
+}
+
 wait_for_pr_merge() {
   local deadline
   deadline=$(( $(date +%s) + WAIT_TIMEOUT_SECONDS ))
@@ -509,11 +547,22 @@ wait_for_pr_merge() {
 }
 
 run_pr_flow() {
+  local source_head_sha=""
+
   if ! command -v "$GH_BIN" >/dev/null 2>&1; then
     echo "[agent-branch-finish] PR fallback requested but GitHub CLI not found: ${GH_BIN}" >&2
     return 1
   fi
 
+  source_head_sha="$(git -C "$repo_root" rev-parse "$SOURCE_BRANCH" 2>/dev/null || true)"
+  if read_merged_pr_for_head "$source_head_sha"; then
+    echo "[agent-branch-finish] Source branch head already landed in a merged PR; skipping new PR creation and continuing cleanup." >&2
+    if [[ -n "$pr_url" ]]; then
+      echo "[agent-branch-finish] Merged PR: ${pr_url}" >&2
+    fi
+    return 0
+  fi
+
   git -C "$source_worktree" push -u origin "$SOURCE_BRANCH"
 
   pr_title="$(git -C "$repo_root" log -1 --pretty=%s "$SOURCE_BRANCH" 2>/dev/null || true)"
diff --git a/test/finish.test.js b/test/finish.test.js
index a03eaf5..07b3959 100644
--- a/test/finish.test.js
+++ b/test/finish.test.js
@@ -466,6 +466,85 @@ exit 1
   assert.equal(result.stdout.trim(), '', 'agent branch should be deleted on origin');
 });
 
+test('agent-branch-finish cleanup skips duplicate PR creation when the current head already landed in a merged PR', () => {
+  const repoDir = initRepo();
+  seedCommit(repoDir);
+  attachOriginRemote(repoDir);
+
+  let result = runNode(['setup', '--target', repoDir, '--no-global-install'], repoDir);
+  assert.equal(result.status, 0, result.stderr || result.stdout);
+  result = runCmd('git', ['add', '.'], repoDir);
+  assert.equal(result.status, 0, result.stderr);
+  result = runCmd('git', ['commit', '-m', 'apply gx setup'], repoDir, {
+    ALLOW_COMMIT_ON_PROTECTED_BRANCH: '1',
+  });
+  assert.equal(result.status, 0, result.stderr);
+  result = runCmd('git', ['push', 'origin', 'dev'], repoDir);
+  assert.equal(result.status, 0, result.stderr);
+
+  result = runCmd('git', ['checkout', '-b', 'agent/test-pr-merged-head-rerun'], repoDir);
+  assert.equal(result.status, 0, result.stderr);
+  commitFile(repoDir, 'agent-pr-merged-head-rerun.txt', 'merged head rerun\n', 'merged head rerun change');
+  result = runCmd('git', ['push', '-u', 'origin', 'agent/test-pr-merged-head-rerun'], repoDir);
+  assert.equal(result.status, 0, result.stderr || result.stdout);
+
+  result = runCmd('git', ['rev-parse', 'HEAD'], repoDir);
+  assert.equal(result.status, 0, result.stderr || result.stdout);
+  const headSha = result.stdout.trim();
+
+  const { fakePath: fakeGhPath } = createFakeGhScript(`
+if [[ "$1" == "pr" && "$2" == "list" ]]; then
+  if [[ " $* " == *" --state merged "* ]] && [[ " $* " == *" --json state,mergedAt,url,headRefOid "* ]]; then
+    printf 'MERGED\\x1f2026-04-22T21:37:13Z\\x1fhttps://example.test/pr/already-merged-head\\n'
+    exit 0
+  fi
+  echo "unexpected gh pr list args: $*" >&2
+  exit 1
+fi
+if [[ "$1" == "pr" && "$2" == "create" ]]; then
+  echo "duplicate PR creation should have been skipped" >&2
+  exit 1
+fi
+if [[ "$1" == "pr" && "$2" == "merge" ]]; then
+  echo "duplicate PR merge should have been skipped" >&2
+  exit 1
+fi
+if [[ "$1" == "pr" && "$2" == "view" ]]; then
+  echo "unexpected gh pr view args: $*" >&2
+  exit 1
+fi
+echo "unexpected gh args: $*" >&2
+exit 1
+`);
+
+  const finish = runBranchFinish(
+    ['--branch', 'agent/test-pr-merged-head-rerun', '--base', 'dev', '--mode', 'pr', '--cleanup'],
+    repoDir,
+    {
+      GUARDEX_GH_BIN: fakeGhPath,
+      GUARDEX_TEST_HEAD_SHA: headSha,
+    },
+  );
+  assert.equal(finish.status, 0, finish.stderr || finish.stdout);
+  assert.match(
+    finish.stderr,
+    /Source branch head already landed in a merged PR; skipping new PR creation and continuing cleanup\./,
+  );
+  assert.match(
+    finish.stderr,
+    /Merged PR: https:\/\/example\.test\/pr\/already-merged-head/,
+  );
+  assert.match(
+    finish.stdout,
+    /Merged 'agent\/test-pr-merged-head-rerun' into 'dev' via pr flow and cleaned source branch\/worktree\./,
+  );
+
+  result = runCmd('git', ['show-ref', '--verify', '--quiet', 'refs/heads/agent/test-pr-merged-head-rerun'], repoDir);
+  assert.notEqual(result.status, 0, 'agent branch should be deleted locally');
+  result = runCmd('git', ['ls-remote', '--heads', 'origin', 'agent/test-pr-merged-head-rerun'], repoDir);
+  assert.equal(result.stdout.trim(), '', 'agent branch should be deleted on origin');
+});
+
 
 test('agent-branch-finish cleanup succeeds from active agent worktree when base branch is checked out elsewhere', () => {
   const repoDir = initRepo();