In real life yes, they are single thickness, but for modeling purposes, oftentimes a double layer will work just fine. A great example where a double layer will work better is exactly as @Gennaro mentioned when using Ebony as the wale plank. It is very difficult to work with, and hard to bend. So modeler would use a thinner Ebony plank on top of the thicker Pear or linden or lime wood. Obviously, the sim will be hidden between the hull planks.