Duplicate posts

I also had big trouble with duplicate posts. In the end uninstalling the plugin “Feedwordpress advanced filters” helped.
http://www.weblogmechanic.com/plugins/feedwordpress-advanced-filters/

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!

https://wordpress.org/plugins/feedwordpress/