Unique profile field?

[markuswarren]

I'm no expert but doesn't the registration screen do this on the username? If so then you could look at the register.php file and see what it is doing for the username. My guess is it's performing a database query on the username field for all user records and if there's a match, inform the user the name is taken. Thus you would do the same query, obviously substituting username for whatever your unique field is.