Duplicate forms

How do I make it where no one can submit duplicate forms based on email address?

Thank you for contacting us. 

Inside of this guide you can find out how to set Form Limits Based on a Unique Question:

http://www.jotform.com/help/223-How-to-set-Form-Limits-Based-on-a-Unique-Question 

You can take advantage of our unique submission feature to block multiple submissions from one system of one IP address by setting the "Loose check" or "Strict Check" respectively. Here is a guide to help you set unique submission in your form:  http://www.jotform.com/help/64-How-to-Set-Unique-Submissions-on-a-Form 

Hope this will help. Let us know if you need further assistance.