How to disable or stop duplicate registrations?

You shouldn't be receiving duplicate submissions, but the issue could be happening if the form is embedded for example and then if the user submits the form and clicks the back button in his Browser.

What you could do is enable unique submissions option:

https://www.jotform.com/help/64-How-to-Set-Unique-Submissions-on-a-Form

That will prevent unique submissions.

Also, you could check this feature: https://apps.jotform.com/app/duplicate_submissions_terminator 

Let us know if you have any further questions.

Thank you!