Q: I may have purchased the same audio book twice?

When making purchases using your Apple ID with the exception of In-App Purchases, I think it's impossible to purchase something twice. For example, if you go to the App Store listing of an app you've already bought, it will display "Open" instead of the price. In which app did you make the purchase? What makes you think you've purchased it twice?

Does it show twice on your account's purchase history : See your purchase history in the iTunes Store - Apple Support ? If it does, and you didn't redownload it (audiobooks are a one-time only download ; if you redownloaded them then you will be charged again), then try the 'report a problem' page to contact iTunes Support : http://reportaproblem.apple.com

Or 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

(I've asked for your post to be moved to the iTunes Store forum, where you've posted is for questions about the iTunes U app.)