Lion dropped the “Save As” option for Apple apps (Pages, Numbers, Keynote, TextEdit, Preview…), and replaced it with “Duplicate”.
Mountain Lion returns the “Save As” option for Apple apps, however with OS X 10.8.0 and 10.8.1 it didn’t work the way you would expect it to. When you did a “Save As”, it saved all changes to the new document AND the original document. This meant if you opened an Apple app document, started making changes, then realized you wanted to save the document with a different name so the original wasn’t changed… you couldn’t.
Apple has fixed this behavior with the OS X 10.8.2 update. Now when you do a “Save As”, you are prompted with a dialog box asking whether you want to change the changes to the original document as well as the new document.