I was charged twice for an in app purchase. How do I get a refund

If both were successful charges (e.g. one didn't fail or is just a temporary authorisation hold), and if you didn't buy them item twice, try contacting iTunes Support : http://www.apple.com/support/itunes/ww (click on your country's flag, the country where you live, and which is therefore the country on the account ; click on Get Support)