What is idempotency and how do you design idempotent APIs?

Idempotency means repeated identical requests have same effect as single request - safe to retry. Essential for: payment processing, order creation, any state-changing operation. Implementation: idempotency keys (client-generated unique ID), store processed keys with result, return cached result on duplicate. HTTP methods: GET, PUT, DELETE naturally idempotent; POST needs explicit handling. Storage: Redis with TTL, database table. Response: return same result for duplicate request. Critical for reliable distributed systems.