'Validate that the email is not already registered with another user'

def clean_email(self):

    if (self.cleaned_data[u'email'].lower() == self.user.email):
        raise forms.ValidationError(_(u"You're    already    known    under    this    email."))
    if get_user_model().objects.filter(email__iexact=self.cleaned_data[u'email']).exclude(email__iexact=self.user.email):
        raise forms.ValidationError(_(u'This    email    is    already    in    use.    Please    supply    a    different    email.'))
    return self.cleaned_data[u'email']
