!fedigrow@lemm.ee discusses this type of issues.

Personally I try to avoid Lemmy.world communities if there is an alternative (e.g. I prefer !movies@lemm.ee to !movies@lemmy.world ), but that’s personal

I think that this reflects “real life”.

You can go to a pub, spout some nonsense and cross the road and spout it again. Each time you are likely to have a different audience, but there’s no guarantee that someone else crossed the road with you.

In the fediverse the same is true. Each community is distinct, even if they’re called the same thing and have shared participation.

You can choose to participate in whichever one you want to, or all of them. There’s no karma associated with voting, it’s about visibility in that community.

If you think it warrants a vote or comment, then do. If you don’t, then don’t.

There’s no right answer here. Do what you think will improve the community you are part of.

I follow communities with the same theme in different instances

Granted, I am starting out, but this is something I wouldn’t do. Which I figure would cut down on seeing what you’re describing.

I don’t think there are officiall guidelines. It’s different comminities so if it is relevant and good content, I upvote both in these situations. Usually not posting the same comment, but depends.