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