Well this is frustrating:

If one edits a document, then chooses 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.

Assuming this is Mountain Lion’s intentional behavior rather than a bug, then how bonkers is this?

So far as I can tell the only real difference between Save As and Duplicate is that the former gives you a dialog box to choose a new name as well as a new location. Whereas choosing to Duplicate a file means only renaming it, and once you do it’s automatically saved in the same location as the original.

Mountain Lion: ‘Save As’ Saves Changes to the Original Document Also