This is not a bug. It's a feature.

Seriously.

Let's say that I have 5 copies of a song from artist X. One is a studio version, the other four are live versions.Each one is unique. Do I want to delete all of them?
No, I don't. I want to keep each one.

Sure, if you buy a ton of "best of ____" cds, you will end up with a ton of dupes, but what are you doing buying THOSE cds :P