The GPG keys listed for the “CentOS / Red Hat Enterprise Linux 6 – atomicrocketturtle.com” repository are already installed but they are not correct for this package.

Atomic are moving everything under one key. To fix this error run this command;

rpm --import https://www.atomicorp.com/RPM-GPG-KEY.atomicorp.txt

Then edit /etc/yum.repos.d/atomic.repo

change the following line

gpgkey = file:///etc/pki/rpm-gpg/RPM-GPG-KEY.art.txt

To

gpgkey = file:///etc/pki/rpm-gpg/RPM-GPG-KEY.atomicorp.txt
         file:///etc/pki/rpm-gpg/RPM-GPG-KEY.art.txt