JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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As I clone your JotForm to my account, I see that Loose check is enabled for Unique submission to limit form submissions. This option will enable the form to check cookies for previous submission and doesn't allow duplicate submission from the same browser. You can change this option from Form preferences -> Form Limits tab.
Hope this information helps!