What are you storing your ripped files on ? I use a NAS (running Asset), ripping straight to it, not to another disk first. Editing the metadata after Asset has scanned it tends to produce spurious artwork for me like you describe. I can almost guarentee that this will happen if I edit the metadata after it's been on the NAS for a while. (Note - I do back up the live library before I make any additions or do any editing )
Also check the album artist and artist sort fields in your metadata - make sure they match the album artist and artist. I had a brain freeze when editing the other day and put "Winwood, Steve" in the sort fields for Suzanne Vega - that produced amusing results with duplicated and/or wrong artwork until I realised and corrected (I had just spent 6 hours editing sort fields to get the A-Z listings how I wanted them - she was quite close to the end of the session).