diff --git a/.github/workflows/governance-reusable.yml b/.github/workflows/governance-reusable.yml
index a8f24572..2264f199 100644
--- a/.github/workflows/governance-reusable.yml
+++ b/.github/workflows/governance-reusable.yml
@@ -997,6 +997,28 @@ jobs:
           # governance jobs on every PR estate-wide. github.sha resolves to the
           # same merge commit but is always fetchable.
           ref: ${{ github.sha }}
+      - name: Checkout standards for the duplicate-key check
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          repository: hyperpolymath/standards
+          ref: main
+          path: .standards-dupkey
+          sparse-checkout: scripts/check-workflow-duplicate-keys.py
+          sparse-checkout-cone-mode: false
+
+      - name: Duplicate YAML keys in workflows
+        run: |
+          # GitHub Actions REJECTS a workflow with duplicate keys: the run is
+          # `failure` with no jobs, no log and no check run. Nothing else here
+          # can see it, because yaml.safe_load silently keeps the LAST
+          # duplicate and reports success — so the file "parses" and every
+          # other lint passes. Measured 2026-08-05: nine workflows in hypatia
+          # were dead this way, including a CodeQL workflow with zero
+          # successful runs in its entire lifetime.
+          cp .standards-dupkey/scripts/check-workflow-duplicate-keys.py "$RUNNER_TEMP/"
+          rm -rf .standards-dupkey
+          python3 "$RUNNER_TEMP/check-workflow-duplicate-keys.py" .github/workflows
+
       - name: Check SPDX headers + permissions
         run: |
           failed=0
diff --git a/scripts/check-workflow-duplicate-keys.py b/scripts/check-workflow-duplicate-keys.py
new file mode 100755
index 00000000..c872c5cc
--- /dev/null
+++ b/scripts/check-workflow-duplicate-keys.py
@@ -0,0 +1,100 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: MPL-2.0
+"""Reject GitHub Actions workflows containing duplicate YAML keys.
+
+WHY THIS EXISTS AS A SEPARATE CHECK
+-----------------------------------
+GitHub Actions rejects a workflow with duplicate keys outright. The run is
+recorded as `failure` with NO jobs, NO log and NO check run — a red mark on
+the board with nothing behind it to read, and `gh pr checks` shows no row at
+all.
+
+Nothing else in the toolchain can see this, because `yaml.safe_load` SILENTLY
+KEEPS THE LAST duplicate and reports success. The file "parses". Every
+ordinary validation — linters, formatters, our own sweep scripts — is
+structurally blind to it.
+
+Measured 2026-08-05: nine workflows in `hypatia` were in this state, including
+a CodeQL workflow with 18 failures, 12 startup_failures and ZERO successes in
+its lifetime. The repository had never once been scanned by its own scanner.
+
+USAGE
+-----
+    check-workflow-duplicate-keys.py [PATH ...]      # default: .github/workflows
+
+Exit 0 when clean, 1 when any duplicate is found.
+"""
+import glob
+import os
+import sys
+
+import yaml
+
+
+class StrictLoader(yaml.SafeLoader):
+    """A SafeLoader that refuses duplicate mapping keys instead of silently
+    keeping the last one."""
+
+
+def _no_duplicates(loader, node, deep=False):
+    mapping = {}
+    dupes = []
+    for key_node, value_node in node.value:
+        key = loader.construct_object(key_node, deep=deep)
+        if key in mapping:
+            dupes.append((key, key_node.start_mark.line + 1))
+        mapping[key] = loader.construct_object(value_node, deep=deep)
+    if dupes:
+        detail = ", ".join(f"{k!r} (line {ln})" for k, ln in dupes)
+        raise yaml.YAMLError(f"duplicate key(s): {detail}")
+    return mapping
+
+
+StrictLoader.add_constructor(
+    yaml.resolver.BaseResolver.DEFAULT_MAPPING_TAG, _no_duplicates
+)
+
+
+def check(path):
+    """Return a problem string, or None when the file is fine."""
+    try:
+        with open(path, encoding="utf-8") as fh:
+            yaml.load(fh, StrictLoader)
+    except yaml.YAMLError as exc:
+        return str(exc).replace("\n", " ")[:160]
+    except OSError as exc:
+        return f"unreadable: {exc}"
+    return None
+
+
+def main(argv):
+    targets = argv[1:] or [".github/workflows"]
+    files = []
+    for t in targets:
+        if os.path.isdir(t):
+            for ext in ("yml", "yaml"):
+                files.extend(sorted(glob.glob(os.path.join(t, f"*.{ext}"))))
+        else:
+            files.append(t)
+
+    failed = 0
+    for f in files:
+        problem = check(f)
+        if problem:
+            print(f"::error file={f}::{problem}")
+            print(f"FAIL {f}: {problem}")
+            failed += 1
+
+    if failed:
+        print(f"\n{failed} of {len(files)} workflow file(s) contain duplicate keys.")
+        print("GitHub Actions rejects these before any job is created — they")
+        print("fail with no log and no check run. yaml.safe_load does NOT")
+        print("catch this; it keeps the last duplicate and reports success.")
+        return 1
+
+    print(f"duplicate-key check: {len(files)} workflow file(s) clean")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv))