During centos 7 update if process stuck in middle or killed yum process, so you may experience this error if you will again update centos.

Error: Protected multilib versions: zlib-1.2.7-18.el7.i686 != zlib-1.2.7-17.el7.x86_64

To resolve this issue we need to do manual recovery. Before starting this process keep backup of /var/lib/rpm and /var/lib/yum.

Now we can use package cleanup command. This command is the part of yum utils.

yum install yum-utils

Now after installing yum utils, we can use package cleanup.

package-cleanup --cleandupes --removenewestdupes

After running this it will take few minutes and now you would be able to run yum update.

Kindly give your feedback and suggestion in comment box.