'Unique' Validation Requires Database Support

The Unique validation requires database support to do the validation. When the form user enters a value, the validation has to check whether that value was entered before (hence making sure the value is unique ). When the form submissions are saved to a database table, the validation can just check the database table.

Therefore, the 'Save to database' option in the 'form processing options' page should be enabled for the 'unique' validation to work.