old_contents: from django.core.exceptions import ValidationError
  
  
  def validate_email_unique(email):
      EmailNotification = email.__class__
      for site in email.sites.all():
          if EmailNotification.objects.filter(name=email.name, sites__id__in=[site.id]).exclude(id=email.id):
              email.delete()
              raise ValidationError('''
                      EmailNotification already exists with (name={} ,site__domain={})
                  '''.format(email.name, site.domain))
  
new_contents: from django.core.exceptions import ValidationError
  
  
  def validate_email_unique(email):
      EmailNotification = email.__class__
      for site in email.sites.all():
          if EmailNotification.objects.filter(name=email.name, sites__id__in=[site.id]).exclude(id=email.id):
              email.sites.remove(site.id)
              raise ValidationError('''
                      EmailNotification already existed with (name={} ,site__domain={})
                  '''.format(email.name, site.domain))
  
subject: remove site instead of deleting email