Honestly, the Reddit approach is pretty similar. Reddit had /r/gaming and /r/games, for instance, with the two communities offering pretty much the same content. Same thing with /r/baseball as the large baseball subreddit and /r/MLB as a mostly empty subreddit filled with people who figured baseball would use the same naming convention as /r/NBA or /r/NFL. Eventually, one of the ones wins out. We just have to remember that Lemmy communities have two names before and after the period, so while the initial name can be duplicated, the initial name plus the instance cannot.

It's similar to the early internet where site.com was different from site.org.

Really the protocols can develop "trending" type functionality for popularity, and "aggregate" groups (tag-based, explicit lists of groups, whatever) for which sub...lemmy's are basically "the same". lemmy.world/aww, lemmy.aww/aww, etc. Lemmy may not do it, but it's doable.