Save As versus Duplicate Commands in Mountain Lion
File→Duplicate and File→Save As in OS X Mountain Lion serve the same purpose and achieve the same result. The difference is that you’ll find File→Duplicate in apps that support Versions and Auto Save and File→Save As in apps that don’t.
They’re different names for the achieving the same result: Saving a file that’s already been saved with a different name, with the option of saving the renamed file in a different folder.
You may be wondering why you would want to save an existing file with a different name? So here’s a good (albeit kind of rude) example: Suppose that you have two cousins, Kate and Nancy. You write Kate a long, chatty letter, and save this document with the name Letter to Kate.
Later, you decide that you want to send almost the same letter to Nancy, but you want to change a few things. So you change the part about your date last night and replace all references to Kate’s husband, Kevin, with references to Nancy’s husband, Norman. (Aren’t computers grand?)
So you make all these changes in Letter to Kate, but you haven’t resaved this document yet, and although the document on your screen is actually a letter to Nancy, its filename is still Letter to Kate.
Think of what would happen if you were to save it now without using the Save As feature: Letter to Kate reflects the changes that you just made. (The stuff in the letter meant for Kate is blown away, replaced by the stuff that you just wrote to Nancy.)
Thus, the filename Letter to Kate is inaccurate. Even worse, you might no longer have a copy of the original letter you sent to Kate! The solution? Just use Save As or Duplicate to rename this file Letter to Nancy by choosing File→Save As or File→Duplicate.
If you chose Save As: A Save sheet appears, in which you can type a different filename in the Save As field. You can also navigate to another folder, if you like, and save the newly named version of the file there. Now you have two distinct files: Letter to Kate and Letter to Nancy. Both contain the stuff they should, but both started life from the same file.
If you chose Duplicate: The title bar of the document becomes editable so you can change its name without even seeing a Save sheet. Press Enter or Return and the renamed file will be saved in the same folder as the original.
Or, if you want to save the newly renamed file in a different location, choose File→Move To or click the little triangle to the right of the document’s name, choose Move To in its pop-up menu, and then select a folder to save the file into from the pop-up menu.
Here’s an easier way to get the same result: Before you start, duplicate the document in the Finder (choose File→Duplicate or cmd+D). Rename the copy, and open it. This way, when you’re done making changes, you don’t have to remember to choose Save As; you can just perform your habitual Save.
This approach also protects you from accidentally saving part of the letter to Nancy without changing the file’s name first. So when you decide that you’re going to reuse a document, Save As (or duplicate and rename the file) before you begin working on it, just to be safe.
Versions give you the benefits of Save As without any action on your part, but only a handful of programs support Auto Save and Versions.