The Unique Field Validation setting can be used to ensure that a designated field has unique data entered into it. One example of its use is to limit one submission per Email address.
Open any form and click on the Advanced tab at the top of the builder then choose Restrictions from the options you see there.
The Unique Field settings will be at the top of the Restrictions settings window.
If a user enters duplicate data in the field that has been chosen as the unique field, the Unique Field Error Message will be shown.