You have an application to deploy to the entire domain, but you want to test deployment first to only a few machines.  You create a software deployment policy with the package and machines, and deployment is successful.  You then modify the deployment to include the rest of the machines in the domain.  The deployment runs, but it also redeploys the application even to the machines that already had it during testing.

Since Desktop Central keeps track of software inventory and can determine what is already installed on a machine, there should be an option to choose if you want to re-deploy software to a machine if it already has it installed.  Similar to how a patch does not deploy to a machine even if you do an "Install Patch" configuration and the machine already has that patch installed.