Fix duplicated orders

[jimmyn]

On the checkout page, we generate an order and assign a unique order ID to the payment. When we receive a callback, we retrieve the order ID from the payment, locate the corresponding order, and update its status. How could this process lead to the creation of a new order? We should only update the existing order and never create a new one. The order ID is unique, and an order is generated only once on the checkout page.