How can I prevent duplicate submissions?

How can I prevent duplicate submissions.

Please try adding a captcha field in the form. I would recommend Google re-Captcha widget:

http://widgets.jotform.com/widget/google_recaptcha 

Here's how to add a captcha in your form(s): 

https://www.jotform.com/help/205-How-to-Add-a-Captcha