If you want to make sure that respondents’ input is unique and you don’t end up with duplicated info between submissions, you can enable Don’t allow duplicate values. Select the field you are interested in applying this validation to and in the left side panel under Validation, you can find the Don’t allow duplicate values option.
Only the following fields have this option:
Use case: the most relevant use case is for the Email address field. There are cases in which you might want to have unique email addresses and by enabling this option, you won’t end up with duplicate input. If one email address has been used once, it can’t be used again.
When respondents provide input that already exists in your form submissions, they get the following field validation error: