This cloning is only possible if you lay hands on a significant part of the victim—such as his brain, his heart, or a large portion of his magic power. If the victim is not dead from this, then he is at least diminished while you access his abilities (reflected by a consequence resulting from the attack that allowed you to steal from him, the terms arrived at when taking him out, etc.).
Seems to me that the whole "laying hands on brains and hearts" could be slightly inconvenient to the donor. Not often you leave those body parts flapping for the purposes of some touchy feelies.