I'm all in favor of several communities on the same topic as long as they offer different content
Having the same link or picture shared across several communities is just detrimental to the user experience.
Also, to avoid this, maybe we should have core communities for a specific topic (for instance unions), and then they can fork if there is a need (e.g. what happened with !android@lemmy.world and !android@lemdro.id
OP, as of now, I would just suggest to block communities that are too similar.
I only have one tech community as they seemed to all be the same anyway, I'll reassess in a few weeks if I need to select the other one.
The problem with blocking a community is if you block the one that eventually takes off, you miss out. I am just accepting it for now and assume it will sort itself out.
Another option would be to upvote one and downvote the other, to help speed the process up.