Postmark, a cloud-based email service provider, occasionally sends webhooks to applications multiple times due to various issues in the distributed system. To handle this, Postmark makes a best effort for "at least once delivery," retrying webhooks over several hours before marking them as failures. This approach is chosen over "exactly once delivery" or "fake it," which would introduce additional complexity and potential issues. Idempotency is key to dealing with duplicate webhooks; it ensures that the system remains in the same state after receiving the same input multiple times, regardless of how many times it's applied. By making inputs uniquely identifiable and using techniques like inserting identities into a secondary table during transactions, applications can achieve idempotent qualities, enhancing their resilience in the face of failure and improving scalability.