Why am I getting billed twice for a song download

If you are being charged double the listed price then you can contact iTunes Support via this page : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption (if you are downloading the song twice then in some countries you will be charged again as redownloading is not currently possible in all countries).

Or are adding or changing credit card details on your iTunes account ? If you are then each time that you do so a small temporary store holding charge ($1 or the approximate local equivalent) may be applied to check that the card details are correct and valid and that it's registered to exactly the same name and address as on your iTunes account - it should disappear off your account within a few days or so.

Store holding charge : http://support.apple.com/kb/HT3702