How can I set a unique question on my form?

Is there a way I can prevent one user from using a coupon code more than once?

There's a way to prevent users from using the same coupon code again. Just set that same field as the Unique Question on your form.

To do that, click PREFERENCES at the top of your FORM BUILDER > ADVANCED tab > then select the Coupon code field as your UNIQUE QUESTION.

Complete guide: How-to-set-Form-Limits-Based-on-a-Unique-Question