I'm seeing the following error on CentOS 6.4:
# puppet agent --test Run of Puppet configuration client already in progress; skipping (/var/lib/puppet/state/agent_catalog_run.lock exists)
What should I do about it?
This is AFAIK perfectly normal if you manually run the agent while it's running in the background. Try checking when your agent is supposed to run, and whether it is currently applying a manifest. See How to detect that a puppet run is complete for details.