It's not necessarily a bad thing to have multiple communities for the same topic. Cross-posting content is much cleaner on Lemmy / kbin than on Reddit; if you're subscribed to multiple communities and something is cross-posted between them, it will only show up in your feed once, so you can safely cross-post to your heart's delight without worrying about spamming people.

Having multiple communities gives people options. Maybe one of them will eventually establish itself as the authority on that topic, where people congregate - and that's fine, but even if not, having alternatives means that if something happens to one of them - the instance shuts down, for example, or even just has an outage, or the mods decide to go crazy and start changing the rules or banning people, or an instance defederates, everyone can just adjust their attention to one of the other alternatives. Compare this to Reddit where when users of a given sub disagree with moderator decisions, they have no real recourse but to try and rally under a new sub with no real reach.

Edit: To add to this, if you cross-post something, it shows (on every community you post to) which other communities it was cross-posted to, so this can be a useful way to inform people reading from one community that the other communities exist, so they can subscribe to them, too, if they want to.