What do I do if I've been billed twice for the same app?

Nobody here knows why you were charged twice. Contact iTunes Store support.

http://www.apple.com/support/itunes/contact/

FOR ASSISTANCE WITH ORDERS - iTUNES STORE CUSTOMER SERVICE

For assistance with billing questions or other order inquiries, please refer to our online support page by clicking here: http://www.apple.com/support/itunes/store/. If you cannot find the answers you are seeking in our robust knowledge base, you can contact us by visiting the following URL http://www.apple.com/support/itunes/store/, clicking on the appropriate Customer Service topic, then using the contact button or email form at the bottom of the page. Responses to emails will be provided as soon as possible.

Phone: 800-275-2273 How to reach a live person: Press 0 four times

Hours of Operation: Mon-Fri: 9am-5pm ET

Email: iTunesStoreSupport@apple.com

How to report an issue with Your iTunes Store purchase

http://support.apple.com/kb/HT1933

iTunes Purchase Problems: How to Report a Problem to iTunes Support

http://tinyurl.com/7tscpa7

How to Get a Refund from the App Store

http://gizmodo.com/5886683/how-to-get-a-refund-from-the-app-store

Getting Refunds for your iTunes Store Purchases

http://www.labnol.org/software/itunes-app-store-refunds/13838/

Canceling a Digital Subscription

http://gadgetwise.blogs.nytimes.com/2011/10/14/qa-canceling-a-digital-subscripti on/

 Cheers, Tom 😉