Make a webhook handler idempotent

The pattern

Use a unique constraint on the event id and let the database reject the duplicate for you, rather than a read-then-write that races under concurrent retries.

async function handle(evt) {
  try {
    await db.insert('processed_events', { id: evt.id });   // unique PK
  } catch (e) {
    if (isDuplicateKey(e)) return;   // already handled — no-op
    throw e;
  }
  await applyEffect(evt);            // your business logic
}

Why not read-then-write

Two concurrent retries can both read "not seen" before either writes, and both process the event. The unique-insert pattern makes the database the arbiter, so exactly one wins.

Retention

You only need to remember ids long enough to cover the 72-hour retry window. Prune older rows to keep the table small.