Yeah these are all great points. It would definitely need to be workshopped to see if there are solutions to some of this or if other approaches are better.
My first thought would be that the top duplicate in the feed would be the one that would be shown (so based on up/down votes I guess). The others would be lower in the list anyway so you would only see those duplicates after the top one if they were there. Simply filtering them out and maybe having a way to show a list of duplicates on the visible one might be good. Then you could choose the comment section of the smaller duplicates of you want to read more comments or whatever. Still lots of unknowns but I feel like it could possibly work.