Q: Why they charge me double for the same purchase if I only bough it one

If you've been charged twice (and one of them isn't a temporary store holding charge, does it show twice on your purchase history ?) then try the 'report a problem' page to contact iTunes Support (these are user-to-user forums) : http://reportaproblem.apple.com

If the 'report a problem' link doesn't work then you can try contacting 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