when you say suplicate files, do you mean duplicate as in content, or as in id3 tags? if the latter, then im sure most music players can do that, itunes is quite good for searching
iTunes is great for handling lots of files, I have about 13,000 songs and it does a super job at managine all of them. iTunes also has a built in feature for finding duplicates so that you can erase the copies u don't want.
i have over 6 days in itunes. if i put that amount of music in wmp it would die a painful death. i dont know if wmp11 fixed the gay database, but wmp10 certainly cannot handle large amounts of music.