Speaking from a similar experience - I expect at some point the "Auto scan all media * - Start" button has been hit and you have not gone through the list of results it collected... So, when you scan for a new cover, it just adds it to the list of ones to be reviewed.
You may be able to clear the stored results without using the admin interface somehow, but I don't know how (maybe someone else can tell you).
If you want an immediate (but slightly annoying fix) click through all the 38 pages, leaving the "None of this" radio selected and click the Match Cover Art button at the bottom of each page - this will clear them all out. After you have done this once - all should be back to normal (assuming you don't hit the "Auto scan all media" button again)