Hi there,
Thanks for reaching out to us, I replied to you in a support ticket, but I just wanted to share it everyone since I think it might be relevant to this discussion.
I looked into your campaign and it seems that this is happening because of where your canonical tags are pointing, you can see the duplicate pages by clicking on the number to the right side of the link. These pages are considered duplicates because their canonical tags point to different URLs. For example:
http://www.virtualsheetmusic.com/score/PatrickCollectionFlPf.html?tab=mp3(Duplicate 1) is considered a duplicate of
http://www.virtualsheetmusic.com/score/PatrickCollectionVcPf.html?tab=mp3 (Duplicate 2)
because the canonical tag for the first page is CANON1(http://screencast.com/t/tqvDZrLsyz8D) while the canonical for the second URL is CANON2 (http://screencast.com/t/FOguPJmK0).
Since the canonical tags point to different pages it is assumed that CANON1 and CANON2 are likely to be duplicates themselves.
Here is how our system interprets duplicate content vs. rel canonical:
Assuming A, B, C, and D are all duplicates,
If A references B as the canonical, then they are not considered duplicates
If A and B both reference C as canonical, A and B are not considered duplicates of each other
If A references C as a canonical, A and B are considered duplicated
If A references C as canonical, B references D, then A and B are considered duplicates
The examples you've provided actually fall into the fourth example I've listed above.
Hope that helps,
Best,
Peter
SEOmoz Help Team.