Having two Addon Manager entries, doesn't mean you have two Addon Manager installed, it only means that Windows has two registry keys that points to an uninstaller, this is likely due to having manually removed some file related to it in the past without running the normal Uninstaller.
It's just a cosmetic issue that doesn't have any effect on the sim. However, if you want to clean it up in the proper way, remove BOTH entries, then reinstall the Stand-Alone Addon Manager again.