Before: Click "save as" > update name = two files.
Now: Click "duplicate" > wait for the duplicate to appear over the old window > click "save" > update name = two files.
Something like twice as many steps are required, now.
My understanding is that it is similar to how Time Machine works, except it is storing the difference between two versions instead of an entire copy. But the active file doesn't contain the history of all of the changes that made it what it is. That history is stored elsewhere in the disk volume. This isn't like Microsoft Word's "Fast Save" or whatever it was called where you could find evidence of previous revisions of the file. That said, if you want to keep a completely self-contained snapshot of your file at a given point, duplicate the file and name it accordingly.