You can configure your event so that a participant cannot register more than once with the same email. By default, attendees are allowed to use the same email address for multiple registrations, but you can disable this feature.
Where can I manage registration authorizations?
Choose your event and click on the "Settings" tile.
Select the "Process" tab
Limit an email to one registration
Uncheck the box "Allow the same email to subscribe several times" to limit the number of registrations per email.
Once the option is disabled, participants will see an error message if they attempt to register more than once with the same email.