I followed Devise's instructions for using usernames in addition to emails for logins. Works great except people can signup with duplicate usernames. If someone tries to signup with a duplicate email address, they are presented with this error message on the signup page:

1 error prohibited this user from being saved:

Email has already been taken

I want something very similar for usernames. How do I implement this?

ruby-on-rails ruby-on-rails-3 devise ruby-on-rails-3.2
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