Preventing duplicate usernames

I've been trying to remember how to prevent duplicate usernames being added when users register / update their profile. Its working in my registration page, but realised I haven't applied it in the profile update page.

Am I right in thinking I should be able to apply a Unique Field Value validation somewhere?

I've been looking by highlighting the field in the form, and selecting:

WebAssist > Forms > Validation > Server Validation

And then Validation Type.

But I don't see Unique Field Value - am I looking in the right place?

Also, how does this work when a user updates their profile? When a new person registers, as long as the username they choose does not already exist in the users table, it will be OK.

But if an already registered user is updating their profile, then their username will exist in the table. In that scenario, will the form update OK if fields other than the username change, but the username does not?