Originally Posted by Sweetpea
Would that mean I'd have to re-download all books again?
Only if you want to use the new option for books already on your device. You must check a box to turn on using UUIDs as file names, and then the option has effect only for books sent after you check that box.
It is true that after checking the option, resending a book over the wireless device connection will create a duplicate. If "On Device" is checked then you should delete the existing copy when you send a new one. The easiest is to send the new copy, notice that it now says "2 books", then delete the older copy.
CC's content server connection already checks if a book with the same UUID is already on the device, and if so overwrites that book. The reverse is not true: CC+calibre do not check for duplicate UUIDs when using the wireless device connection. And yes, I agree that it is strange that one pathway checks while the other doesn't, but we didn't want to make CC's calibre device driver work differently from the other calibre devices. And in any event, such a check does not solve the problem where the same book can have a different file name on different devices.