I don't think this is a thing and I'm not sure it reasonably can be.
- Maybe if someone properly crossposted, Lemmy could know which posts are identical and skip dupes. Though it would still be a crapshoot which community got displayed... you might end up seeing the comments from the original post in some tiny/dead community while a crosspost to a huge community blows up with it's own comments.
- But for non-crossposted duplicate posts... there's no relationship between them as far as lemmy is concerned. They're separate posts to separate communities that just happen to look very similar. Deducing such a scenario is very sticky.