How Idempotency Keys Prevent Duplicate Charges and Errors in ASP.NET Core APIs

When a client retries a failed HTTP request, side-effecting API endpoints can process the same operation twice, leading to duplicate payments, repeated notifications, or corrupted inventory data. Idempotency keys — unique identifiers generated by the client per logical operation — allow servers to detect and skip already-processed requests by returning a cached response. Developers can implement this in ASP.NET Core 8 using middleware that checks a distributed cache like Redis before executing any business logic. The cached response is stored for a set duration, typically 24 hours, so all subsequent retries receive the original result without re-triggering backend operations. This pattern is especially critical for financial transactions, order creation, and third-party integrations, while read-only endpoints generally do not require it.