Mountain Lion: ‘Save As’ silently overwrites old document

Data Loss via 'Save As':

If one edits a document than does Save As, then BOTH the edited original document and the copy are saved, thus not only saving a new copy, but silently saving the original with the same changes, thus overwriting the original.

Just tried it with a TextEdit document and it indeed saves both documents with the new contents. Not really the expected behavior.