diff --git a/.github/workflows/gate.yml b/.github/workflows/gate.yml
index 7ea8a24ad..a1ac78851 100644
--- a/.github/workflows/gate.yml
+++ b/.github/workflows/gate.yml
@@ -764,6 +764,32 @@ jobs:
       - name: Run audit-section-33-migration-xrefs (--enforce)
         run: bun tools/hygiene/audit-section-33-migration-xrefs.ts --enforce
 
+  lint-backlog-id-uniqueness:
+    # Fail if any B-NNNN ID is claimed by more than one backlog row file.
+    # Cross-agent ID-allocation collisions (Otto-CLI vs Otto-Desktop on
+    # B-0444 2026-05-13; Lior vs Otto-CLI on B-0532+B-0533 2026-05-15) cost
+    # ~15 min coordination effort each at observed ~20% rate. The
+    # duplicate-ID detection logic was added to audit-backlog-items.ts
+    # 2026-05-14 (PR #3249, Copilot caught two files claiming B-0329 on
+    # PR #3247) but ran detect-only — this job is the CI gate that turns
+    # codified-but-not-enforced into a control.
+    #
+    # B-0535 — sibling of lint-section-33-migration-xrefs + lint-archive-
+    # header-section33: catch-once-then-lint pattern at ID-allocation scope.
+    name: lint (backlog ID uniqueness)
+    timeout-minutes: 2
+    runs-on: ubuntu-24.04
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      - name: Install toolchain via three-way-parity script (GOVERNANCE §24)
+        run: ./tools/setup/install.sh
+
+      - name: Run audit-backlog-items (--enforce-duplicate-ids)
+        run: bun tools/hygiene/audit-backlog-items.ts --enforce-duplicate-ids
+
   lint-no-empty-dirs:
     # Fail if a committed directory has no files — almost always a
     # forgotten artefact (an agent-created skill folder without a
diff --git a/tools/hygiene/audit-backlog-items.ts b/tools/hygiene/audit-backlog-items.ts
index 00b1d5faf..0c8f3a1b4 100644
--- a/tools/hygiene/audit-backlog-items.ts
+++ b/tools/hygiene/audit-backlog-items.ts
@@ -34,11 +34,14 @@
 //      PR #3247; PR #3249 added this audit class).
 //
 // Usage:
-//   bun tools/hygiene/audit-backlog-items.ts
+//   bun tools/hygiene/audit-backlog-items.ts                       # detect-only
+//   bun tools/hygiene/audit-backlog-items.ts --enforce-duplicate-ids
+//       # exit non-zero on duplicate-ID groups (B-0535 CI gate)
 //
 // Exit codes:
-//   0 -- survey ran (findings reported in body)
-//   1 -- fatal invocation error (e.g., backlog dir missing)
+//   0 -- survey ran (findings reported in body); detect-only mode
+//   1 -- fatal invocation error (e.g., backlog dir missing) OR
+//        duplicate-ID groups found AND --enforce-duplicate-ids set
 
 import { existsSync, readdirSync, readFileSync } from "node:fs";
 import { dirname, join, resolve } from "node:path";
@@ -601,6 +604,15 @@ async function main(): Promise<number> {
     return 1;
   }
 
+  const argv = process.argv.slice(2);
+  const enforceDuplicateIds = argv.includes("--enforce-duplicate-ids");
+  for (const arg of argv) {
+    if (arg !== "--enforce-duplicate-ids") {
+      process.stderr.write(`error: unknown argument: ${arg}\n`);
+      return 1;
+    }
+  }
+
   const today = nowIso();
   const nowEpoch = Math.floor(Date.now() / 1000);
 
@@ -652,6 +664,13 @@ async function main(): Promise<number> {
   );
   console.log("  (typed-edge backlog graph).");
 
+  if (enforceDuplicateIds && duplicateIdGroups > 0) {
+    process.stderr.write(
+      `\nerror: ${duplicateIdGroups} duplicate-ID group(s) found; --enforce-duplicate-ids set (B-0535 gate)\n`,
+    );
+    return 1;
+  }
+
   return 0;
 }