I think people are worrying about it too much.
On Reddit, there were tons of duplicates. r/tumblr and r/CuratedTumblr. r/animemes and r/animememes and r/goodanimemes. r/gaming and r/games.
For Magic the Gathering there was r/magicTCG, r/mtg, r/magicthecirclejerking, r/MagicArena, r/EDH, r/mtgmemes, r/mtgfinance, r/ModernMagic, r/Pauper, r/mtgbrawl, r/DinosaursMTG, a bunch more, plus probably a dozen more I didn't even know about.
Dungeons & Dragons had r/DnD, r/dndnext, r/onednd, r/DungeonsandDragons, r/DnDMemes, r/3d6, and again, lots more.
If there are enough people in each community, great! More communities! If not, that's still okay! People will naturally gravitate towards more active communities. A lot of the duplicate communities will eventually figure themselves out and one will become the "main community." We just have to wait a bit. For now I'm just subscribing to anything that matches my interest to see what happens with it.