Unique profile field as the name suggests, provides an option to make the profile fields created by the profile (core) module, unique.
After installing and enabling, this module will provide an option to make any profile field unique on its edit page. This is generally at admin/user/profile/edit/%fid .
If this option is checked, the module will let users only enter unique values for that field. The check for uniqueness is performed on the registration page (if any profile fields are used on the registration form); and when a user tries to add or update general profile information.
This project is similar to unique field. However, unique field does not provide an option for making profile fields unique. Which is where the inspiration for this project comes from.
Possible use-case: A company that lets user's register against a Unique identifier provided by a third party: like a social security number or a PAN number. This field then has to be unique.
There are two possible alternatives to using this module:
1: Use the username as a unique field for registration and other purposes. However, this limits you to using only one field as unique.