I just installed 2.1 tonight, and I'm positive this was not a problem before.
If I file a post under more than one category, I now get more than one post. A post in two categories spawns a twin. Three categories, three posts. They're identical in appearance and URL and are stacked one on top of another. If I cut down to one category per post, the duplicates vanish; they also return if I put the multiple categories back. This happens to both old and new posts.
EDIT: The duplicates don't seem to actually exist; they don't show up on the Manage Posts page, at least.