Blur a face using only one video layer with Premiere CS6?
Yes and no, you can do it without duplicating the clip, but not without a second layer.  Instead of using a second copy of the clip, you can use an adjustment layer.  You make a new adjustment layer by creating a New Item in the Project window.  You add that adjustment layer over top of the video you want to adjust and then you apply your masking to the adjustment layer and apply your effects to the adjustment layer.

The effects applied to the adjustment layer are then applied to the sum total of all the layers below the adjustment layer.  Effectively, the adjustment layer is a layer that is composed of a sub-render of everything below it and then has the set of effects applied to it applied to that sub-render.

It technically isn't adding two copies of the sequence, but practically it is still doing the same thing.  I'm not personally aware of any more straightforward technique than that for doing what you describe in CS6 though.  That doesn't necessarily mean much though as I normally do this kind of thing in AE anyway, so my experience doing it in Premiere is limited at best (bordering on non-existent.)