I realise this is a known issue and that lemmy.world isn't the only instance that does this. Also, I'm aware that there are other things affecting federation. But I'm seeing some things not federate, and can't help thinking that things would be going smoother if all the output from the biggest lemmy instance wasn't 50% spam.
Hopefully this doesn't seem like I'm shit-stirring, or trying to make the Issue I'm interested in more important than other Issues. It's something I mention occasionally, but it might be a bit abstract if you're not the admin of another instance.
The red terminal is a tail -f of the nginx log on my server. The green terminal is outputting some details from the ActivityPub JSON containing the Announce. You should be able to see the correlation between the lines in the nginx log, and lines from the activity, and that everything is duplicated.
This was generated by me commenting on an old post, using content that spawns an answer from a couple of bots, and then me upvoting the response. (so CREATE, CREATE, LIKE, is being announced as CREATE, CREATE, CREATE, CREATE, LIKE, LIKE). If you scale that up to every activity by every user, you'll appreciate that LW is creating a lot of work for anyone else in the Fediverse, just to filter out the duplicates.
When I've mentioned this issue to admins at lemmy.ca and endlesstalk.org (relevant posts here and here), they've suggested it's a misconfiguration. When I said the same to lemmy.world admins (relevant comment here), they also suggested it was misconfig. I mentioned it again recently on the LW channel, and it was only then was Lemmy itself proposed as a problem. It happens on plenty of servers, but not all of them, so I don't know where the fault lies.