docs: def clean_email(self):
          """ 
          Validate that the email is not already registered with another user.
          
          """
          if self.cleaned_data['email'].lower() == self.user.email:
              raise forms.ValidationError(_(u"You're already known under "
                                                  "this email address."))
          if get_user_model().objects.filter(
                  email__iexact=self.cleaned_data['email']).exclude(
                      email__iexact=self.user.email):
              raise forms.ValidationError(_(u'This email address is already '
                          'in use. Please supply a different email address.'))
          return self.cleaned_data['email']