Can I add a form to a site and make sure there's no duplicate signups?

Hi there! I'm we want to create a website for a promotion for a car wash. We've been using QR CODES for the past promotions we've done, however, we haven't gotten much success through them. I'm wondering if there is a way where people will be driving past a sign on our Car wash that tells them to go to a website with a form on it, but allows them enter their Email ONLY ONCE. So I guess what I'm asking you is if there's a way to prevent duplicate signups with the same email?

Please contact me as soon as possible!

Yes it is possible.

You can select one of your fields, like email address, in order to accept only once the same value. In order to do so please go to Preferences > Form Limits > Unique question:

Then you can select one of your form's text-boxes. 

Let us know if we can assist you further.