How Duplicate Webhook Deliveries Expose Idempotency Gaps in Backend Systems

A developer discovered idempotency flaws while building a personal project, when the same webhook event was processed multiple times due to provider retries after lost responses. In distributed systems, a webhook provider may retry delivery if it does not receive a confirmation, causing the same business logic to execute more than once. The core principle of idempotency requires that repeating an operation produces the same outcome as performing it once, preventing duplicate side effects. A common fix is storing processed event IDs to skip duplicates, but simultaneous duplicate requests can both pass the check before either records the event. This race condition means a simple check-then-act pattern is insufficient, and atomic or database-level locking mechanisms are needed for true idempotency.