If this has happened, we're very sorry. It's a mistake and we'll fix it.
Did you receive 2 confirmation emails?
If you did, then it seems the order was placed twice. You can cancel one via "Your Account" - you'll find a link to your account at the top of every page.
If you only got one email, then it is most likely we had a problem when you tried to place your order and the failed attempt has not been refunded yet. We'll automatically refund payments that are not linked to orders.
If you are still convinced you have an outstanding double charge, please let us know and we'll investigate further.