first make sure the files aren't really duplicated. I found some that were actually on two of my drives (C drive and my Music hard Drive). Check the paths.
Also check first to make sure the same song isn't on 2 different albums that you have loaded. (Regular album and Best Of!) You can delete the duplicate songs if you need space. Remember some "Best Of" versions might be a slightly different version. Also you should keep the one that is the best recording!

Ok, all this for if they are true duplicates, but if it is just the data base relax;

if it is truly a duplicate of the same track, don't fret, it is a known issue, but doesn't cause any real problems.
Go to WMP help page on Microsoft to read how to clear the cache, it's pretty easy.
It happened to me when I switched out my back up music drive for the first stand alone hard drive (I do this once in awhile so they both get equal hours of use, non use.)
After you clear the cache it will rebuild the data base. of course this can take time depending on how much you have stored.