Buy by mistake of game.

You can try going to http://commerce.microsoft.com to see if you can start a refund there. If not, you'll need to contact support and speak to billing. 1-800-4my-xbox if you're in the US/Canada or http://support.xbox.com/contact-us

First mistake buying MW3.

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Second mistake opening the game.

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Third mistake complaining about a copy and paste game.

For purchasing issues you would need to contact support. (800) 469-9269 if you're in the US/Canada or
http://support.xbox.com/contact-us

You'll have to contact Xbox Support for assistance about a refund

your first mistake was buying the game, your second mistake was playing it problem solved