How can I warn the user that the same email address has been used before submitting the form?

How can i warn that the same email address has been used to submit this form before right at the beginning rather than after submit??

Secondly, how can I enable multiple submissions with the same email address?

Unfortunately, there is no option to display the warning before submitting the form when enabling the Unique Question/Submission limit. It is always displayed when the form is submitted.

To enable multiple submissions from the same email address, you would need to disable the Unique Question or Submissions limits in the form settings.

https://www.jotform.com/help/223-How-to-set-Form-Limits-Based-on-a-Unique-Question

https://www.jotform.com/help/64-how-to-set-unique-submissions-on-a-form