Checkout Logic Bugs

Scenario

Coupons like SAVE50 should apply only once per checkout session. The server never tracks whether a code was already used on the same cart. Stack the same discount twice on one order.

Your approach

1. Start checkout with a single valid coupon applied.
2. Apply the same coupon code again through the API or form.
3. Confirm the total drops further than a single use allows.
4. Read the coupon reuse flag on success.
5. Submit that flag.