My games are all over the place and I shamefully admit I not once, not even twice bought a game on a sale on one platform only to realize I already owned it on another. Oopsie.
Steam and GOG are my main platforms for sure, Origin/uPlay/Others are only used when there's no other choice. Luckily such times are rare. Oh right, Battle.net is different as it only serves as my gateway to StarCraft II ladder. The only platform I don't use is probably Epic Games Store - after my "journey" with Metro Exodus and seeing Epic throwing money at everything without any regard to actual customers, I am boycotting this platform entirely.
Maybe I should make a spreadsheet with all the games I have. GOG Galaxy 2.0 seems pretty nice tho, and I am a lazy bum :?