If you want to reject duplicate input values for a form field, you can easily do that by enabling the option Don’t allow duplicate values, which is found in Form Editor → Edit Field.
The most relevant use case example is that of unique email addresses. There are situations in which you don’t to accept duplicate email addresses. If
firstname.lastname@example.org has been used once, it can’t be used again.
When form users provide input that already exists in your form submissions, they get the following field validation error:
Please enter another value. This one already exists in this form's submissions.
The option of rejecting duplicate entries can be used for the following form fields: Short Text, Number, Email, Website.