What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
I have submitted your form and then I inspected your submissions and mine submission was not duplicated. However I did notice duplicate submissions but they do not have the same time. It is possible that your users have clicked back on your Thank you page and submitted your form again.
Inside of this article you can find out how to set form limits based on a unique question:
This way you can limit your users to have only one submission per email address.
Hope this will help. Let us know if you need further assistance.