Getting rid of Save As… in Lion and adding the Duplicate command was, in my opinion, a mistake. Fortunately, Mountain Lion makes it easy enough to get our old friend Save As… back with the Option key.
If you add two shortcuts to your Keyboard preference pane, you can get Save As… back as the default, and only show Duplicate when holding down Option. Here is how to configure this:
And here is the File menu after making the change:
Keep in mind this will also change the “Duplicate” shortcut of all apps to Command+Option+Shift+S. So for example, in your IDE or image editor of choice, Duplicate will no longer be accessible via Command+D. You can get around this by setting shortcuts for each individual app, but that seems like a pain to manage. For now I’ll stick with Save As… back. It really depends on which feature you will use more: Duplicate or Save As.