Script to conditionally cancel ongoing request and return custom error in user registration

I need to make a user custom field unique just like the e-mail field is unique. Is there a way to do that?

If it can't be done..I need to verify if this custom field already exists in the users table and then if it exists I have to return a custom error message and stop the registration.

At least if someone show me how to programatically cancel the request it would be a beggining to me.

Thanks.