How missing idempotency keys cause duplicate charges on API retries

Short summary

When a payment API response is lost and the client retries, the backend may process a second valid charge—creating duplicates for orders, emails, subscriptions, and more. The fix is an idempotency key: a unique value per logical operation that the client generates and reuses across retries. The server stores the operation state and original response in a PostgreSQL table so retries receive the same outcome without re-executing the business action.

• •Retries are only safe when the server can distinguish repeated operations from new ones
• •Idempotency keys tie multiple network attempts to one business action
• •Server must store operation state, response body, and fingerprint to safely replay results

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