How idempotency keys prevent duplicate charges and data errors in APIs

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Idempotency is a design property that ensures retrying the same API operation produces the same result as executing it once, preventing issues like double charges or duplicate records. The problem arises with POST requests, which are not naturally idempotent, meaning a network timeout followed by a client retry can trigger the same action twice. To solve this, clients generate a unique key per intended operation and include it with each request; the server stores the key alongside the result and returns the cached response on any repeat. Critical implementation rules include saving the key and its result atomically within the same transaction, using a database uniqueness constraint to block concurrent duplicate processing, and scoping keys per account to avoid collisions between users. Keys should also carry a request body fingerprint to reject reused keys with different parameters, and a time-to-live should be set to keep storage manageable.

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