docs: def _check_data_flow_id(self, data_flow):
          """Checks the validity of a data flow id
  
          Checks whether the id of the given data flow is already by anther data flow used within the state.
  
          :param rafcon.core.data_flow.DataFlow data_flow: The data flow to be checked
          :return bool validity, str message: validity is True, when the data flow is valid, False else. message gives
              more information especially if the data flow is not valid
          """
          data_flow_id = data_flow.data_flow_id
          if data_flow_id in self.data_flows and data_flow is not self.data_flows[data_flow_id]:
              return False, "data_flow_id already existing"
          return True, "valid"