How do I stop a MonCash payment from being fulfilled twice?

Question

Sometimes an order gets marked paid twice and we ship twice: the customer refreshes the return page, and our background checker also finds the payment. How do I make fulfilment safe?

Accepted Answer

Make "mark as paid" **idempotent at the database level**, not just in application code. Two requests arriving at the same moment will both pass an `if (order.status !== 'PAID')` check.

**1. Store the MonCash transaction id with a unique constraint**

~~~sql
ALTER TABLE payments ADD CONSTRAINT payments_moncash_tx_unique UNIQUE (moncash_transaction_id);
~~~

**2. Transition the order with a conditional update**

~~~ts
async function markPaidOnce(orderId: string, transactionId: string) {
  return db.$transaction(async (tx) => {
    // only one caller can move PENDING → PAID
    const { count } = await tx.order.updateMany({
      where: { id: orderId, status: "PENDING" },
      data: { status: "PAID", paidAt: new Date() },
    });
    if (count === 0) return false; // someone else already did it

await tx.payment.create({ data: { orderId, provider: "MONCASH", moncashTransactionId: transactionId } });
    return true;
  });
}
~~~

Only the caller that gets `true` sends the confirmation email or ships. Everyone else just shows "already paid".

**3. Use one `orderId` per payment attempt.** If a customer abandons and retries, create a new attempt with its own id that links back to the order. MonCash rejects a duplicate `orderId`, and you get a clean audit trail.

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