How Do I prevent a duplicate registration

How Do I prevent a duplicate registration

On the settings page of your form, you have to select a form field from Unique Question. Unique question will work on any input type such as Email fields or plain Text fields. You example if you set the Unique Question to the email field, the form will prevent duplicate submission with the same email address. 

Inside of the guide on which you posted your comment there is a step by step explanation how this can be done. Please check it again: 

How to set Form Limits Based on a Unique Question

You can also use Unique Submission feature to prevent duplicate submissions.

How to Set Unique Submissions on a Form