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.
Since this issue is most prevalent while browsing new, you could temporarily store (a hash of) the titles and contents of each post during a given browsing session, and if a post matches another post on the same page (e.g. one set of posts from Everything/new), hide it and allow the user to see comments from each instance of the post when they view the canonical (earliest, or most active, or returned first) post.
If another duplicate post is later loaded (through scrolling) the post could be hidden and replaced with a smaller indicator telling the user a duplicate post has been detected, and they can click it to view the comments.