the really confusing part for me, or i guess the part that makes me believe he is a clone, is if you choose the Dark Side ending,
spoiler: when you are about to strike down Vader, another clone of Starkiller ends up killing you. Vader gets up saying that the cloning process has been perfected in the clone that had just stabbed you, and you were just a pawn to get the Rebels to come to Kamino, where he could eliminate them all
Personally my choice of ending, the light side ending was to "happy go lucky" for me, Starkiller is a tortured soul, there will be no happy endings for him.