How Idempotency Keys Prevent Duplicate Requests in APIs

Idempotency ensures that repeating the same API request multiple times produces the same outcome as sending it once, preventing issues like duplicate charges or double-sent emails. While HTTP methods like GET, PUT, and DELETE are naturally idempotent, POST requests require an explicit mechanism — typically a client-generated unique key sent via a request header. The server stores this key alongside its response, returning the cached result if the same key is received again. A basic implementation can use in-memory storage, but production systems should rely on Redis or a database with unique constraints to handle race conditions atomically. PostgreSQL's conflict-handling on inserts is one reliable approach to guarantee that only one request is processed even under concurrent retries.