I just got a new MacBook Pro (hallelujah), and when I was coping over my music library from my IBook it was about 9GB, but I just noticed that my music library is now about 18GB. So I started opening some of the folders in ~/Music and I noticed that are multiple copies of the same file.
So now do I know get my music library back down to 9GB? I was thinking about dragging and dropping my music onto the desktop, then deleting everything in ~/Music, then adding everything back to my library; but I don't know if that would work or not.