How to prevent users from submitting multiple entries?

Hello. I need a form for research, but it can not be answered twice by the same person. Is it possible to create a registration mechanism that can block the same people from responding twice? Registering for example the email?

Yes, it's possible to set a unique submission or allow only single user from submitting the form. Please check our guide here https://www.jotform.com/help/64-How-to-Set-Unique-Submissions-on-a-Form for steps.

Let us know if you have any questions or if you need further assistance.