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Purchases are considered final but you could try the 'report a problem' page to contact iTunes Support and see if they will cancel it as a one-off : http://reportaproblem.apple.com
Or you can try contacting iTunes Support via : https://support.apple.com/contact or https://www.apple.com/emea/support/itunes/contact.html
If they do : View, change, or cancel your subscriptions - Apple Support