I accidentally downloaded the same book twice can I get my money back?

A refund may not be possible since the terms of sale for the iTunes Store state that all sales are final. You can contact the iTunes Store, explain the reason for your request, and ask, though:

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

It's possible they'll make an exception for you. They often will in the case of mistaken duplicate purchases, though it's not guaranteed.

Good luck.