Occasionally (quite frequently), when fetching all feeds, articles from one feed will appear (duplicated) as unread articles under some other feed. And these are not new articles fetched in this cycle. Old articles under some feeds suddenly appear duplicated as unread articles under some other feed. This is happening since the last couple of days.
The problem seems to have rectified after several rounds of OS package updates.
*** This bug has been marked as a duplicate of bug 458116 ***