I also once tried to buy a book twice and Amazon wouldn't let me. It said I had already downloaded it.
Also just to clarify something that everyone keeps saying, that Amazon will keep a copy of all the books that you buy from it so that you can redownload them. I suppose most of you know that this is almost certainly not really true. There's no way they keep a copy of your books. I'm sure they just keep a list of the books you downloaded, and then when you request one, they go to the online book and download it. That is why you can get an updated version of any book. I assume most of you know this, but just in case there was any confusion for the new Kindlers, I thought I'd be more explicit.
Steve