No Duplicates option allows you to validate data uniqueness. Please, see below a simple example of how to use this validation option.

1) Create a project with form.

2) Add just one field to the new form with a Single line type.

3) Open the field editing page.

4) Find Validation tab on  nd click on it. Check data uniqueness in the column and click Update button. See screenshot below:

You can enter the error message.