docs: 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"))