On the Users screen if you enter data that is the same as another user the error messages that come up are vague and give no indication to what the problem is.
The errors seems to be consistent between creating and editing - you'd see the same error for the same duplicate data if you are creating a new user with that data or editing an existing user.
When a duplicate email address is used you receive the error message, "Cannot apply changes: resource is already in use".
When a duplicate username is used you receive the error message, "Cannot apply changes: Object namespace collision".
