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/usr/lib/python2.7/site-packages/yum/yumRepo.py
    def _groupCheckDataMDNewer(self):
        """ We check the timestamps, if any of the timestamps for the
            "new" data is older than what we have ... we revert. """

        if 'old_repo_XML' not in self._oldRepoMDData:
            return True
        old_repo_XML = self._oldRepoMDData['old_repo_XML']

        if (self.timestamp_check and
            old_repo_XML.timestamp > self.repoXML.timestamp):
            logger.warning("Not using downloaded repomd.xml because it is "
                           "older than what we have:\n"
                           "  Current   : %s\n  Downloaded: %s" %
                           (time.ctime(old_repo_XML.timestamp),
                            time.ctime(self.repoXML.timestamp)))
            return False
        return True

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2017:2295