Personally, I don't understand the need to buy multiple copies. I know some people tend to say it's so they can have one for the car, one for home, etc. which kind of makes sense.
I'm the opposite though, I refuse to buy more than one copy of an album if the majority of the tracks are identical. Which has caused me a lot of frustration with certain artists (Beyonce, Rihanna, Toni Braxton, etc.) who re-release with a few even better songs that I would have wanted. I guess I could download them, but I still listen to most of my music on CD and would find it annoying to have to find the song on my computer everytime I want to listen (I also don't have any kind of mp3 player, nor do I have any interest in one). Generally now, I wait until I find out if there's going to be a rerelease or deluxe edition before I buy anything. It's annoying, but rereleases are too common now.
The only reason I would ever buy an album I already have is if the disc was damaged to the point where it wouldn't play. If there was a better/deluxe version, I'd replace it with that. But then, I think I'd still have some issues with that (I need to replace my first 2 Harry Potter books because they're getting worn out, but every time I stop myself and think "Well, you already have it...")