How would you implement idempotent notification sending to avoid sending a duplicate email if a background job is retried?

Similar to job idempotency discussed earlier, notification sending should track which specific notification (identified by a unique key, like 'order-confirmation-{orderId}') has already been sent, checking that record before sending and skipping (or treating as a no-op) if it's already been dispatched -- this prevents a retried job (due to a worker crash after the email was sent but before the job was marked complete) from sending the same email to a user twice.

async function sendOrderConfirmation(orderId) {
  const alreadySent = await db.sentNotifications.findOne({ key: `order-confirmation-${orderId}` });
  if (alreadySent) return; // idempotent no-op
  await emailProvider.send(buildConfirmationEmail(orderId));
  await db.sentNotifications.insertOne({ key: `order-confirmation-${orderId}`, sentAt: new Date() });
}