You don’t see things correctly, imo. In FMOD, the “Mixer”, though it’s called so, isn’t the equivalent of the mixer of your DAW. The equivalent of your whole DAW project is a single event in FMOD. The equivalent of the mixer of your DAW in FMOD is (inside an event) the tracks controls, effects and sends (that you generally configure in the bottom tab), busses being “return tracks” you can create as you want, all being eventually routed to the master track of the event.
FMOD Mixer is indeed a mixer, but one (or more) level(s) above the event. Its purpose is to route and mix all your different events themselves. Sending individual tracks of one event to different mixer groups isn’t the normal way to use it, that’s why we should use the workaround I told you.
As I said, it’s a workaround. But the trick is to put the main volume track to min (-inf), so no, there won’t be duplicated audio. The send is in fact used as a routing.