If I know I'll be using the Patch Tool, I'll do it on a duplicate layer, because it doesn't work on a blank layer (at least, not in my version).
If it's only going to be a small area of cloning, I'll usually do it on a blank layer.
There's no rule about it. Whatever you feel comfortable with. Obviously duplicating the layer makes your file much bigger.
A couple of things to read, here and here, if relevant.