You built a Shopify integration. It works great. Until it doesn't. A customer's network drops right after the checkout POST fires. They refresh. The browser retries. Shopify receives two identical order creation requests. You now have a double order, a double charge, and a very confused customer. This is an idempotency problem. And it is more common than most Shopify developers expect. What Is Idempotency in a Shopify Context? An operation is idempotent if running it once or a hundred times produces the same result. In Shopify integrations, this covers: • API mutations (order creation, fulfillment, refunds) • Webhook processing (order.paid, inventory.update) • Background job retries • Third-party sync operations Strategy 1: Use Idempotency Keys on REST API Calls Shopify's REST Admin API accepts an Idempotency-Key header on POST requests. Generate a UUID v4 per logical operation and send it with the request.…