Hello LJDawson, let me start with saying how happy I am to have Sync back :)
The one issue I have with Lemmy is duplicated posts between different communities (see screenshot). Posts usually have same title, author and link, yet differ in community/server they are posted to. It would be great to be able to make the appear as one item on the feed with some note below that links to both instances' posts, or something like that basically to make them not appear twice or thrice one under another.
I feel like this should only be a problem when users don't make use of the crosspost functionality built into Lemmy (which ultimately should only be a technical problem that Lemmy clients need to address at the time of post creation). Aside from appropriate options in a client that should allow you to crosspost to multiple communities at once, I feel like this is a server side issue more than a client side one because the server shouldn't be returning duplicates like this if posts are appropriately crossposted IMHO.
Well, the comments are separate for each crosspost so I'd expect the server to return all crossposts equally...
I don't really know the best way to deal with that on the server API side, but I don't think it's to return them all as separate posts in the main feed. This is a problem that could clearly get much worse before it gets better because it could make the All/New feed really painful to read.