Duplicate username already taken error messages

Created by: bbodenmiller

When I try to add a user in the admin dashboard, and the username is already taken, I see two similar notices:

• Username has already been taken
• Username already exists

Steps to reproduce

Go to admin Create user with username: john.doe Try to create another user with username: john.doe

Similar issue as duplicate email message: https://github.com/gitlabhq/gitlabhq/issues/6123