How Duplicate Webhook Deliveries Silently Corrupt Payment Data

Payment providers like Stripe, Midtrans, and Xendit routinely retry webhook events, but most applications are not built to handle these retries safely. Common failure modes include duplicate event delivery, out-of-order processing, malformed payloads, mid-process timeouts, and partial commits followed by failed downstream calls. A developer built a local test lab with 18 deterministic tests to reproduce these failure scenarios and validate fixes without involving real transactions. Key recommended safeguards include verifying HMAC signatures against the raw request body, storing both event IDs and payload hashes to detect suspicious retries, enforcing strict state machine transitions, and capping automatic retries at a fixed number before routing events to a human-review queue. Together, these measures address the subtle, compounding bugs that typically cause payment integrations to fail in production.