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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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"