I buy everything multiple times. I always buy albums at midnight on iTunes & then get them in the store the next day too. Recently, I've been re-purchasing stuff because I'm really OCD & want everything in my library to be iTunes purchased and I'm deleting the CD rips.
Although if we're just talking about physical CD's that I've bought multuple times, These are some of them.
Gwen Stefani - The Sweet Escape
Fergie - The Dutchess
The Black Eyed Peas - Monkey Business
Miley Cyrus - Breakout
Selena Gomez & The Scene - A Year W/o Rain
Kelly Clarkson - Breakaway
Ke$ha - Animal
Christina Aguilera - A Decade Of Hits (I lost this one like twice & replaced it both times.)