How idempotency keys stop duplicate charges when you click Pay multiple times

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When a payment page freezes and users click the Pay button multiple times, each click sends a separate network request to the server, risking duplicate charges. To prevent this, payment systems use a mechanism called an idempotency key — a unique ID generated the moment a user initiates a charge, not each time a request is sent. This key is attached to every retry attempt for that same transaction, so the server can recognize repeated requests and process the charge only once. The key is tied to the user's intent rather than individual network attempts, meaning retries from app crashes, slow connections, or impatient clicks all carry the same identifier. Both the client and the server must cooperate in this system, with the client reusing the same key on retries and the server using it to detect and ignore duplicate requests.

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