The default behavior will be backward-compatible, still permitting reinstallation.

In module.xml, an application that wants to suppress reinstallation of an already-installed version could have:

      <Defaults>
        <Parameter Name="PermitReinstall">0</Parameter>
      </Defaults>

At the command level, a user trying to install such an application when they really know better can say:

zpm "install isc-supply-chain -DPermitReinstall=1"

If PermitReinstall=0 and you run the install command, rather than erroring out, there should just be a message saying " is already installed."