Duplicate Killer – This extension prevent double entry submissions for Contact Form 7, Forminator and WPForms Lite plugins.

Duplicate Killer will prevent double entry submissions for Contact Form 7, Forminator and WPForms Lite plugins.
This plugin can be used to ensure that a designated field has unique data entered into it, the best example of its use is to limit one submission per Email address.
Choose the unique fields of your forms from Email, Phone, TextField.

Create your form with Contact Form 7, Forminator or WPForms plugins.
Find the Duplicate Killer display page through the WordPress admin interface
Click on the tab which suits you(Contact Form 7, Forminator or WPForms tab)
Choose your unique fields based on your form configuration (Name, Phone, Email and TextField is supported)
Set your custom error message for users when value has been already submitted
Done

Duplicate Killer is an extension of Contact Form 7, Forminator and WPForms plugins.
Unique values is inserted into database when user submit a contact form from the plugins listed above.
Duplicate Killer will check and throw an error when the value submitted by user is already in database.
This way will ensure that your contact form contain only unique values.
Unique values won’t be able to be submitted by any another user.

This feature use cookies and prevent duplicates at the user level, not global.
Multiple users can submit the same entry, but a single user cannot submit values they have already submitted before.