If you don't want to allow your users to possibly use the same code twice during check out create a set of unique codes and upload them! Here is how to do that (troubleshooting tips on the bottom):
1. Head to your design canvas
2. Highlight your "coupon code" area and head to the settings area for the code
3. Once the options area is open you'll find a unique code area. There you can copy and paste your items or upload the files! If you run out don't worry, we send you a friendly reminder that you're running low.
Troubleshooting: If your looking to cycle out old codes for new ones this can be a little tricky. You must upload the new codes and at the same time check the "purge existing coupon" button THEN his save changes This will cycle out the new codes for the old ones. If you simply try to purge the codes hit save and then try to upload codes it will not work.