Why duplicate it? Duplicating a layer hides the original layer unless your intent is to blend the two layer in some way. You also do not want to work on every area of the layer. IMO you would be better off just adding a new empty layer and heal and clone into to that layer to correct imperfections. Perhaps use frequency separation what you cak edit coror on one laye and texture on an other.
As for sharpening you normally was to sharpen the whole image for output. That can be done by adding a sharpening adjustment layer on top. While Photoshop does not have a sharpen adjustment layer feature it is quite easy to create a sharpening adjustment layer using Photoshop tools. Once you have created the sharpening adjustment layer you can tweak it. Because of that I normally ever sharpen then tweak the sharpen by painting on the sharpening adjustment layer layer mask changing its blending opacity and fill. Since I do that so often I created a sharpen adjustment layer action years ago the last time I update that action was back in CS3.