I had the same issue here. All my rss feeds are custom made (build by myself or i use feed43.com and do validate. The feeds did not change before/since the upgrade of Multi-importer). But then duplicate posts started to occur. I checked the url and title’s of the duplicates (as registered in the database), and they are all identical (!!) So in theory, this should not happen.
I have about 15 rss feeds. It worked fine until the upgrade.
Most posts are kept ‘pending’ (and never published, I had around 200 post marked as pending).
I just removed all the rss feeds, disabled the feed to post setting. Moved all pending posts to trash. Re-entered the rss feeds again and activated the feed-to-post setting.
For now it works. New rss feeds (I forced some to update) are recognised and existing (unmodified) feeds are ignored.
I will watch the behaviour of the plug-in the next few days, if there are any problems, i will post a message.
Cheers,
Koen