class Case(models.Model):
  ...

  def save(self,*args,**kwargs):

    if Consumer.objects.get(email__iexact=self.email):
        self.consumer = Consumer.objects.get(email__iexact=self.email)

    else:
        consumer = Consumer(first_name=self.first_name,last_name=self.last_name,email=self.email)
        consumer.save()

    return super().save(*args,**kwargs)