Audit my checkout flow end to end for every path that can produce a duplicate order or charge, and close each one. Cover: the pay button (disable on submit and show working state, as UI courtesy, not as the defense); the server endpoint (one pending order per checkout session, so a second request joins the existing attempt instead of creating a new order); the payment provider call (idempotency key forwarded so provider-level retries don't double-charge); the success redirect (page refresh after payment must not re-run the charge; the confirmation page reads order state, it doesn't trigger payment); and the webhook handler (order fulfillment keys off payment provider events, processed once per event, not off the redirect alone, since users close tabs before redirects complete). Don't change the checkout UX flow. Don't treat the client redirect as proof of payment; only a verified webhook or a server-side status check against the provider confirms money moved. Deliver: each duplicate path found and the fix, and a stress pass: double-click pay, refresh mid-checkout, close the tab after paying and return, refresh the success page. Exactly one order and one charge in every case.