How Idempotency Keys and Redis Prevent Duplicate API Charges in Node.js

In real-world networks, dropped connections and automatic retries can cause non-idempotent POST endpoints to process the same request multiple times, leading to issues like duplicate payments. Idempotency keys are unique client-generated identifiers sent in HTTP headers that allow servers to detect and safely handle repeated requests. When a server receives a keyed request, it stores the processing state in Redis and returns a cached response for any subsequent retries, skipping re-execution of business logic. A concurrent duplicate request received while the original is still processing is rejected with a 409 Conflict response to prevent race conditions. A lightweight Node.js middleware using ioredis can implement this pattern with atomic Redis operations, offering production-grade exact-once execution semantics for critical API routes.