Duplicating Layers in Photoshop
I duplicate layers in Photoshop when I'm about to make some major changes, I duplicate certain layers to keep a safe version of the original, then if I make a mess of the copy I can refer back to the original version. You can of course use the history palate to step back but if you are using eg hundreds of brush strokes the history will only go back so far.
Open the image up in photoshop, and select the layer that you want to duplicate.
Click and hold the photoshop layer and drag it to the bottom section of the layers palate on to the new layer button, highlighted in red in the image on the left.
Now you will have two versions of the same layer, you can rename the layers so you know which is which, or just turn off the visibility of the safe layer as shown on, by clicking on the eye symbol box and delete the layer that you dont want.