@EltaninPub: Well, depending on your save template, it might very well be, since it probably won't end up with the exact same path/filename, but you'll probably end up with two copies of the same book: one being the original from Amazon, and another one, sent by Calibre.
(And even with the same filename, in-place metadata editing of a book already cataloged by the Kindle should work fine).
But, that's nitpicking

. I completely agree that you should delete the 'original' book after importing it into Calibre, if for no other reason than uncluttering the documents folder

.