580 / 1048 · Module 12 — Idempotency and Duplicate Prevention← prev⊞ allnext →12.51Test 4 — same idempotency key, different payloadRequest 1: REQ-123 amount=100 Request 2: REQ-123 amount=900 Verify: second request rejected if that's your contract. ← Previous12.50 Test 3 — ambiguous timeoutNext →12.52 Test 5 — concurrent duplicate messages