I’m still on the train of “duplicate communities will sort themselves out if community discovery is made much easier and popular communities reliably show up at the top community searches, mostly irrespective of what instance the search was performed on”

Honestly, we've all seen this happen in every community that fragments for some reason. Whenever reddit banned a subreddit, 1,000 mini-subs would spawn, eventually coalescing into 1 or 2 main communities. Whenever a migration happens (Tumblrinos moving to reddit, redditors moving to various reddit-like things, forums moving to reddit, fark migrations, usenet, IRC, etc) they always fragment initially only to end up with a relatively few coalesced groups that hold similar ideas.

c/news might end up with 10,000 news subreddits (even one for each local newspaper!) but eventually it comes out to something like: c/news, c/leftnews c/rightnews c/ihateallnews with the most active (by far) being one of the generic ones. The others might technically exist, but if you search "news" and get one that has 10,000,000 subs and another that has... 3, well, you're just not going to bother.