Build-an-X for idempotency tests in any async/job/API context - idempotency-key handling (per Stripe / AWS prescriptive guidance pattern), retry-safe semantics (exactly-once vs at-least-once vs at-most-once), side-effect commutativity verification, fingerprint-based dedup, idempotency-window tuning. Use when authoring tests for any system where the same input could be processed twice (SQS Standard at-least-once, RabbitMQ requeue, retry-on-error logic, webhook redelivery, browser double-click, mobile-network retry).