Thanh Nguyen • 11/5/2025

Webhook Idempotency: Lessons from a ‘Double Charge’ Production Bug

This article recounts a real production incident where a payment system's webhook handler caused double fees due to a race condition and lack of idempotency. It explains webhook delivery guarantees (at-least-once), defines idempotency, and presents three implementation patterns to ensure exactly-once processing. The piece also covers webhook security, testing strategies, and best practices for building resilient systems. Aimed at developers and engineers working with payment integrations, it provides practical lessons and code examples to avoid similar bugs.

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#Production Bug #Payment Systems #At Least Once Delivery