Replay-Safe Webhook Dedupe in n8n: First Event Only

Replay-safe event ingestion for n8n webhooks—accept only the first delivery

This n8n automation workflow receives HTTP POST events, validates their event envelope, and deduplicates them using a source:event_id key so replayed or duplicated deliveries are safely rejected while the first one is accepted.

What this workflow does

• Receives POST webhooks at an n8n webhook endpoint using the “Receive Event” trigger.
• Validates the JSON body: it checks required fields such as event_id (or eventId) and source (or origin), then normalizes them.
• Builds a deduplication key in the format source:event_id.
• Handles invalid requests safely: if validation fails, it returns HTTP 422 with an invalid_event_envelope error payload.
• Deduplicates using n8n history: it checks whether the dedupe key has been seen in prior executions.
• Accepts only the first event: for first-seen keys, it returns HTTP 202 confirming acceptance and includes event_id, source, and dedupe_key.

Use cases

• SaaS operators ingesting third-party webhook events that may be retried or replayed during network issues.
• Automation engineers building reliable webhook consumers where idempotency is required.
• Event-driven systems that need predictable behavior across retries within a replay window.

Technical details

• Nodes/logic: webhook, if, code, remove duplicates, respond to webhook, plus a sticky note for guidance.
• Dedupe method: relies on n8n’s built-in deduplication history (configured size is currently 10,000 keys).
• Setup: copy the webhook URL from the “Receive Event” trigger, ensure stable event_id/eventId and source/origin, and adjust endpoint path/method/response codes if needed.