Regarding your last paragraph, I agree. I'm subscribed to gaming in lemmy.ml and beehaw so see the same content twice regularly. Duplicate communities raise other concerns for me though:

Which one is the defacto community to join? Using the Gaming community as an example, maybe one leans more to images and the other has more meaty discussion threads just by way of who has joined those communities - nothing to do with the rules. But if you subscribe to both, the majority of the content may be duplicate posts instead? It's not clear from the community title alone.

Is the potential squandered as communities are potentially splintered? Maybe people just stick to one community without joining the other. It'll take time for a certain community to establish itself as the main community with the highest quality posts, but due to the volume of users on the main instances maybe there won't be a main community? Or maybe people won't even be aware of multiple communities for the same topic as the names are different, e.g. football Vs soccer.