Itunes file twice the size as library

when you add music to your itunes library, it copies them to the itunes music folder for itunes to manage it or itunes has every song twice in its library, to check whether itunes has not copied your music twice, view your music in LIST VIEW in itunes and if you see some songs has been copied twice delete on of them.
You can also delete the songs on your music folder to save disk space or you can leave them if you use another music player other than itunes.
But I think you should delete the files in your music folder.