A bank or payment provider sends a webhook the moment something happens. A transfer completes, a payment settles, a dispute is opened. The backend receiving that webhook is supposed to update its records and move on. Simple in theory.
In practice, the same webhook often arrives more than once. The provider's server times out waiting for a response, assumes the delivery failed, and sends it again. Now the backend has processed the same event twice. If that event triggered sending a confirmation email, the customer gets two emails. If it triggered releasing funds or updating an account balance, that is a much bigger problem, and the kind of bug that does not show up in testing, only under real load with real retries.
If I walked into a bank or fintech company and found this happening, this is exactly how I would think through fixing it using NestJS.
Why this keeps happening
Most payment providers, from Stripe to Worldpay to bank transfer rails, use what is called at least once delivery. They do not promise to send an event exactly once. They promise to keep retrying until they get a successful response. This means duplicates are not a bug on their end, they are expected behavior on your end that has to be handled.
