what is Idempotency?

The ability for an operation or code to be run multiple times without any unintended or extraneous effects. For example, if on your configuration management platform, you have it set so your web servers will install Apache, Apache will only ever be installed once, regardless of how many times the configuration manager is run against a server in that group.

Most configuration management tools are idempotent, including Ansible, Chef, Puppet, and Salt.