Ensure the email address is not already registered.

def clean_email(self):
        """
        Ensure the email address is not already registered.
        """
        email = self.cleaned_data.get("email")
        qs = User.objects.exclude(id=self.instance.id).filter(email=email)
        if len(qs) == 0:
            return email
        raise forms.ValidationError(
                                ugettext("This email is already registered"))