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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.