You can apply the No Duplicates setting to an individual field to prevent people from entering an answer that has already been submitted by someone else in a previous entry.
If someone enters an answer that already appears in another entry, we'll show them an error message that says "This field requires a unique entry and this value was already submitted." They won't be able to submit your form successfully until they enter a unique value into this field.
It's best to use this feature when you have a field on your form whose answers should most likely be unique for each and every person filling out your form—things like name, address, student ID number, etc.
To apply the No Duplicates setting to a field:
You can apply the No Duplicates setting to the following field types: