I also had big trouble with duplicate posts. In the end uninstalling the plugin "Feedwordpress advanced filters" helped.
In my case the duplicates where always created from the first new post in the aggregated feed. It was copied as many times, as new items in the feed where present. :)
So if I got 10 new items, the first item was copied 11 times, but only 1 time it was recognized as an syndicated post, all other posts looked like regular posts.
Hope this helps!