docs: def is_valid(self):
          ''' Validate form.
          Return True if Django validates the form, the username obeys the parameters, and passwords match.
          Return False otherwise.
          '''
          if not super(AddUserForm, self).is_valid():
              return False
          first_name = self.cleaned_data['first_name']
          last_name = self.cleaned_data['last_name']
          if User.objects.filter(first_name=first_name, last_name=last_name).count():
              non_field_error = "A profile for {0} {1} already exists with username {2}." \
                .format(first_name, last_name,
                        User.objects.get(first_name=first_name, last_name=last_name).username)
              form_add_error(self, '__all__', non_field_error)
              return False
          if self.cleaned_data['user_password'] != self.cleaned_data['confirm_password']:
              form_add_error(self, 'user_password', "Passwords don't match.")
              form_add_error(self, 'confirm_password', "Passwords don't match.")
              return False
          return True