Could it be something to do with an instance running multiple servers, i.e. for horizontal scaling? Maybe there’s some configuration problem where two servers are sending out the federation rather than just one.
Obviously if you only run one lemmy process (which I think most smaller instances do) then it’s not a problem. But if you run more (which I’d assume lemmy.world does) then it could perhaps be the reason.
EDIT: Probably should’ve checked your links first as that seems to be exactly what is happening lol
I’m only running one process, I’d assume the problem isn’t happening for Feddit.dk.