.tip

Providers retry when your endpoint is slow, and a retry is the same event again. Charging a customer twice is the usual outcome.

INSERT INTO webhook_events (id) VALUES ($1) ON CONFLICT DO NOTHING RETURNING id;
-- no row returned: already processed, respond 200 and stop

Insert the event id first, in the same transaction as the work. Respond 200 fast and do the slow part from a queue.