The Zip is slightly unique in that there is both a Music menu and a Card menu. By selecting the Card menu, you can browse files stored only on the external memory card. The Music menu however, will display songs located both in the internal memory and on the card as well. It is not a listing of songs located exclusively in the internal memory location.
Is this what you are seeing that leads you to believe that the files are copied to both locations? As Bob indicated, as you browse through the list in the Music menu, the files actually located on the card will have a small icon next to them.
Note also that you can go to the bottom of the Music menu to Folders. Here you can select either Internal Memory or External Card and navigate to verify what files are residing where.
