The geniuses at Apple decided that there should be a difference between Duplicate and Save As. One the surface most people don’t see the difference between the two options. Both create a new copy of the same document which you can then edit. However, with the addition of previous versions there is a subtle difference. At the end of the day, most people will find a method the like (I still stick to the Save As option). In this post I will try and explain the differences. Note that you access the Save As menu option by holding down alt.
When you use the duplicate menu two things happen. The first is an exact copy of the document you are working on appears. The second is that this is an unsaved copy that you have to manually save when you finish editing it.
This means you can create a copy (a duplicate), change it mess around with it. If however, you don’t like your changes you don’t have a file that you have to find and delete.
Using Save As
The Save As menu option work in the opposite way to the duplicate menu option. It creates a copy, however you have to save this file up front (different from saving at the end). It also makes this duplicate copy the only active copy. The original file is still stored.
When To Use Duplicate or Save As
One the surface they are similar however it is the subtle differences that make them different. The duplicate menu is something you use if you want to mess around with a file, then decide if you want to keep it. The Save As option is what you use if you want to create a backup, or an alternation but want to keep those changes.
With the use of previous revisions and the mini time machine, both are slightly unneeded as you can always go back in time. There is no need to make duplicates.