When there is rpm duplicates error like below while doing upgrade, it is either ctrl-C was hit while upgrading or there is connection problem(system crashed or reboot) therefore leaving the upgrade incomplete. New packages might be installed, but old packages are not removed.
yum-utils-1.1.30-30.el6.noarch is a duplicate with yum-utils-1.1.30-17.el6_5.noarch nspr-4.9.5-2.el6_4.x86_64 is a duplicate with nspr-4.9.2-1.el6.x86_64 nss-3.14.3-4.el6_4.x86_64 is a duplicate with nss-126.96.36.199-12.el6.x86_64
Solution 1: yum-complete-transaction --cleanup-only yum clean all yum update
Solution 2: package-cleanup --cleandupes yum clean all yum update