Your form accepts multiple submissions by default. If you want to prevent your form submitters from making multiple submissions then here are your options.
Don't Check — This option means the form will not check for duplicate submissions.
Use Cookies — This option is a loose check. When you choose this option, the form will check whether the submission came from the same browser. It uses cookies stored in the user’s browser to verify duplicate submissions. Resubmissions are prevented by tracking cookies on the user's browser. Note that this is not a fool proof way of ensuring unique submissions. The form submitter can delete the cookie and can submit the form again.
Use IP address — This option is a strict check, which means the form checks whether the submission came from the same browser and IP Address. Please note if you select this option then people belonging to the same network such as school and colleges might not be able to make submissions.