code: def is_correct(self):
          
          state = True
  
          # Make members unique, remove duplicates
          if self.members:
              self.members = list(set(self.members))
  
          if self.unknown_members:
              for member in self.unknown_members:
                  msg = "[%s::%s] as %s, got unknown member '%s'" % (
                      self.my_type, self.get_name(), self.__class__.my_type, member
                  )
                  self.add_error(msg)
              state = False
  
          return super(Itemgroup, self).is_correct() and state
docs: Check if a group is valid.
          Valid mean all members exists, so list of unknown_members is empty
  
          :return: True if group is correct, otherwise False
          :rtype: bool