[Clipper]

Duplo-Ray works like the card Mate from Fantasy, that you will find many more rulings on. Those rulings all state that a Mate is a different monster and can be affected by cards as a single monster, but as the card is an exact duplicate of the other monster, you usually affect the Mate rather than the monster so it's easier to keep track of things.

In the case of Rock of Repentance, this would move the Duplo-Rayed monster to fight on the good side, while the monster would remain on the monster side. Effectively, they would cancel each other out (both monsters adding the same combat strength to their respective sides, unless the original monster gets a bonus against one of the munchkins). The battle will continue.