Use creates to simulate idempotency for command actions

The command action by default will run each time its invoked in a playbook (and each time the playbook is invoked). However you can make command idempotent by using the creates attribute.
For instance as part of an automated mysql install using ansible I run mysql _install _db to seed the initial databases. Rather than run that each time the playbook is called I check to see if the mysql database directory already exists (implying it has already run)

action: command /usr/bin/mysql_install_db --datadir ${datadir} creates=${datadir}/mysql