There are so many duplicates because you're generating items with different GUIDs for the same thread. Google Reader doesn't just present whatever's in the feed at that instant, it caches older items, and (correctly) uses the GUID to identify unique items.
If the feed is going to do that you could make it more useful by putting the new post text in the body of the item, not the original post text.
I preferred it the old way where it only included new threads, and I could subscribe to the thread if I was interested.
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