How payment systems prevent double

Short summary

Payment systems prevent duplicate charges through defense-in-depth: frontend button disabling, backend idempotency keys, database unique constraints, distributed locks, and payment gateway idempotency. Idempotency keys ensure repeated requests with the same payment ID return the cached result instead of reprocessing. Each protective layer is essential because attackers and network failures can bypass frontend safeguards.

• •Idempotency keys recognize duplicate payment requests and return cached results instead of reprocessing
• •Multiple defensive layers (frontend, backend, database, Redis locks, payment gateway) ensure single charge
• •Real payment providers like Stripe and Razorpay implement idempotency at the gateway level

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