unwanted duplication of songs

> I have a bunch of songs in my Media Player and they are indexed two ways; by
> artist and by songs under the artist. In the past I have downloaded backup
> DVD's or CD's with all the same existing songs but with some new songs also.
> All I've wanted to ever do is just add the new stuff into Media Player;
> however, I usually end up downloading it all back into Media Player, old
> existing songs along with the new stuff..........the problem is that when I
> do this, Media Player fails to recognize the existing songs already in place
> and instead duplicates all the same songs I already have in there, and as a
> result I end up with duplications of songs in my library. I would have
> thought that the system would realize an existing song already in the library
> and would not duplicate it, but that's not what happens.
> Is there anything I can do to stop the duplication?
> thanks