Checks whether there exists another job which has the same job key.
        :param job: The job to check.

        :return:    True if there exists another job with the same key, False otherwise.

def has_job(self, job):
        """
        Checks whether there exists another job which has the same job key.
        :param job: The job to check.

        :return:    True if there exists another job with the same key, False otherwise.
        """
        job_key = self._job_key(job)
        return job_key in self._job_map