Which approach should be used to ensure a customer order is processed only once?

Setting the Idempotency-Key header to the order ID is an effective approach to ensure that a customer order is processed only once. The Idempotency-Key is specifically designed to prevent duplicate processing of requests that could occur if a user resubmits a payment due to network issues or timeouts. By associating the Idempotency-Key with the order ID, Stripe can recognize that an order with that specific ID has already been processed and will ignore any additional requests with the same key. This mechanism ensures that regardless of how many times the request is sent, the order will be processed only once, thus preventing any unintended duplicate charges or order processing.

The use of a unique transaction code alone does not inherently prevent duplicates, as it does not provide a mechanism for the server to recognize repeat requests. Requiring manual approval for orders adds friction to the process without directly addressing the potential for duplicate processing. Enabling multi-step verification can enhance security but does not specifically target the issue of duplicate order processing either. Therefore, using the Idempotency-Key is the most reliable method to safeguard against processing the same order multiple times.