In my post on how to Double Process a Single Raw File I showed the two different ways of creating a copy of a Smart Object (SO) layer. To summarize, if you simply duplicate the layer both SO layers are “linked” back to the same version of the raw file. And any changes you make to one will be reflected in the other. But if you choose New Smart Object via Copy then you create a second independent SO. The resulting 2 SO layers can be modified independently from one another.
On his Adobe Photoshop Killer Tips site Matt Kloskowski has a short video that shows the differences in these two ways of making a copy of a SO layer. If my explanation leaves you scratching your head, check out his 2 minute video.