How do you prevent duplicate triggers across systems?

In distributed systems, duplicates are the default state. Webhooks get retried after timeouts. Sync jobs run with overlapping windows. The same purchase event fires twice because of a retry. Without explicit prevention, your email automation will eventually trigger the same sequence twice for the same person.

The primary defense: idempotency. Design your event processing so that receiving the same event twice produces the same result as receiving it once. This usually means:

Unique event IDs. Every event should have a unique identifier. Usually a UUID generated at event creation, included in the webhook payload. When your endpoint receives an event, check whether you've already processed that event ID. If yes, return a success response and do nothing. Most webhook delivery systems include an event ID field; if yours doesn't, generate one before sending.

Deduplication windows. If unique IDs aren't available or consistent, track events in a time-bounded window (30-60 minutes is common). If you see the same event signature, same contact, same event type, same timestamp range, within that window, treat it as a duplicate.

State checks before action. Before triggering an automation, check whether the contact is already in that sequence or has already received those messages. This catches duplicates that slipped through ID-level deduplication.

The layered approach is more reliable than any single method. Event IDs catch most duplicates. State checks catch the rest. Neither alone handles every case.

If you're using a tool like Zapier or Make, check whether your trigger has a built-in deduplication option, most do. If you're building custom webhook handling, log every incoming event ID to a database table and check before processing. The setup takes an hour; the duplicates it prevents can take days to untangle. Good API design assumes retries will happen, plan for them.