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 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.