Here's what I know about this issue.
Duplicate Post Plugin does not work.
Duplicate posts only appear for me when Feedwordpress is operated in automatic mode. When I do the manual updates by clicking the button never do I have a problem with duplicate content.
New version appears to be buggy in that it will automatically update for a day or two and then stop until a manual update is done.
It is all very frustrating. There is no consistent reason as to why, how or where from duplicate posts come. Some days it is one blog that we syndicate, some times a week goes by with no duplicate posts, and then another blog will have a post show up five or six times.
Obviously, at least as far as I am concerned, this is a problem with the method the program uses to automatically update as it never happens in manual mode. Manual mode however is just annoying to use and I wish it would get fixed.
I keep saying that if this program ever gets rid of the bugs I will donate a couple hundred dollars to the author because it is so useful for what I do. But every release seems to have the same problems.