The error message does not make it clear, but you do not need to install IPAM on your additional pollers. IPAM only runs on your primary polling engine.
I had the exact same question and contacted support to verify (case 443338). The support representative also referenced a previous case (352421) stating that IPAM does not run on the additional polling engines, only on the primary.
Saying that, I have no idea why there is an installation file for the poller (hence the confusion).
Edit: This has been marked as correct but as Michal Hrncirik points out below, the IPAM poller executable is used to update the Orion Core services on your additional poller. If you have already upgraded/installed another module that has upgraded the Core, you will receive this message about the software already being installed.
