I mean, I keep coming back to that being the entire point of the Fediverse.

Sure, it means in some ways discussion can be different on different servers, but isn't that the entire point? To not have a single point of failure? If one instance puts down draconian rules that makes a certain community unwelcome, then why not move to an instance that you feel is welcoming?

In other words, post where you feel like, and/or where the people are. I really don't have an issue with cross-posting to other instances or having different discussions about the same subject.

Some instances have defederated with others, so another aspect of this is having multiple communities means people who have been defederated can still take part in the same community on a still federated instance.

Just hope one wins and everyone posts there

That's one way.

Fragmentation exists even on centralized systems, though. (How many Dungeons & Dragons subreddits are there? How many are there about cats?) So it's probably best to temper expectations of one forum to rule them all, regardless of what platform we're on.

or is there a technical solution?

We may start seeing fediverse clients that can group related communities together and show them as a single feed, like multireddits.

I don't see it as needing a solution. I can subscribe to five hiking/camping communities across various instances, posts show up in my "subscribed" feed, and I don't really care I'm looking at multiple communities. I care about the topic, not the instance.

I picked an instance that has reasonable rules that I believe will play well federating with other civil instances. Its a feature, not a bug, that I've found multiple communities focused on my hobby.

Keeping the discussions central may be to the detriment of the community IMO

Follow both and see which one you prefer. I follow a few of duplicate communities and most have reached a point now where there's a difference between them, whether it be the type of posts, quality of responses, or moderation policy

I would just subscribe to both and then post on whichever is bigger.

I don't think it's a problem everything doesn't need to be in one place.

Follow both.

Some folks won't be able to access some communities due to defederation, and in other cases folks may want to start a community on their home instance for certain reasons. This isn't the end of the world; probably in the long run either everybody will centralize on the biggest one, or you'll end up with one or two alternatives that have a bit different culture or focus.

That being said, if you like to read comments, it can get confusing when a bunch of different communities all have high interaction posts about the same topic at the same time.

Just follow both and if you decide you prefer one, leave the other. It's the same as on reddit when there are two subs with slightly different names and the same content. One will eventually become larger and people will gravitate towards it.