I still question not the feasibility of such an experiment, but whether or not it would be more rewarding than dangerous. First of all, is sex with a doppelganger the same thing as sex with a clone? Not, right? Let's explore.
A doppelganger, by definition is n. A ghostly double of a living person, especially one that haunts its fleshly counterpart. So a doppelganger could be a surprisingly friendly sexual novelty, like a Real Doll. And it's not a real person, so you don't have to feel bad about raping it - right?
Sex with a clone, on the other hand, I think I would issue an emphatic "no" to. I don't want to know how I am. And if it has my personality, even worse. I don't want to hear my mood-killing jokes and riotous laugh.
The only other detail in question is whether or not your doppelganger is in fact your "evil" counterpart. In a lot of ways this makes it more appealing, since I, like most women, am deeply attracted to a-holes.
So I think it's decided then. Sex with doppelganger, worth trying. But please everyone - use protection. If you and your doppelganger conceive, you'll both get sucked into a dimensional vortex from which you can only watch idly while your progeny brings about the certain destruction of humankind.