I have done it several times. The key is whether or not the old plastic laminate is still in good shape and bonding to the countertop. If it's loose anywhere or it has "bubbles" in it, you would be better off taking the old layer off first.If everything is O.K. then just sand down the old layer of "Formica" and give it a rough finish, then clean the dust off really well to allow the glue to bond to the new Formica. Then lay the new P-lam just like a normal install.Remember, your adding about an 1/8th inch height to the countertop, so keep that in mind.