I haven't had any success with copying and pasting sketches, since they never seem to go where I want them to go, but I have found a better way to work it out. If you have a plane parallel to the original sketch plane, then open a new sketch on the plane where you want the copy. Now, go over to the feature management tree on the left and find the sketch that you want to copy. Select it, and it will change color in your part window. Leave it selected and go over to the command toolbar and pick "convert entities" which is the little cube with one edge darkened. It will create a copy of the original sketch on the new sketch plane. It will be projected directly above the original position.