Retries in distributed systems can silently corrupt data without idempotency. This guide explains how to implement idempotency keys based on IETF standards using Java 21 and Quarkus. It covers the data model, service layer with proper concurrency handling, REST endpoint implementation with fingerprint validation, manual testing with curl, and production considerations like TTL cleanup. The pattern prevents duplicate side effects by storing request fingerprints and replaying cached responses for identical retry attempts, working for both REST APIs and message-driven workflows.