Hi I just purchased twice a movie via iTunes , how can I cancel one

Try the 'report a problem' link to contact iTunes Support and see if they will refund or credit you (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