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.
This is most likely related to your user's end, he/she might have left the submit page of the form in his/her browser, then when every time it opens, the form is resubmitted.
You can directly contact that email and ask if he/she is accidentally re-submitting the form.
It's also possible that this may be a spam, you could set up a captcha on your form field to give it a challenge: https://www.jotform.com/help/205-How-to-Add-a-CAPTCHA-field.
Another way to limit submissions is by using the following features:
Limit submission by cookie or IP: https://www.jotform.com/help/64-How-to-Set-Unique-Submissions-on-a-Form
Limit submission by unique answer on a form field: https://www.jotform.com/help/223-How-to-set-Form-Limits-Based-on-a-Unique-Question
May we also know if the form is embedded on a website? If it is, could you please share us the link?
I hope that helps.