'When signing up make sure the username isn\'t already used by
a different user, and doesn\'t contain invalid characters.'

def clean_username(self, username):

    if ('@' in username):
        raise forms.ValidationError(USERNAME_EMAIL)
    if (not USERNAME_REGEX.match(username)):
        raise forms.ValidationError(USERNAME_CHARACTERS)
    username = super(KumaAccountAdapter, self).clean_username(username)
    if get_user_model().objects.filter(username=username).exists():
        raise forms.ValidationError(_(u'The    username    you    entered    already    exists.'))
    return username
