The Postgres error code that makes Stripe webhooks idempotent: 23505 (opens in new tab)

Stripe delivers webhooks at least once. Not exactly once — at least once. A network blip on your side, a 200 that arrives a half-second too late, an event Stripe decides to re-send during an outage, and the same checkout.session.completed lands on your endpoint twice. Sometimes three times. Your handler does not know that. If that handler grants a subscription, sends a welcome email, or increments a usage counter, "at least once" is how one signup becomes two welcome emails and a double-count...