Starkiller isn't a clone, from what I can gather. He is mentioned on the newer holorecords on the site. You are called subject 1138 on the holocom on the level Empirical on the 1st game, and this same name is mentioned as 'returning' to Kaimino while the klaxons wail and starships scream overhead. I believe that the Galen you play as has been manipulated into thinking he's a clone, while Vader keeps him alive to attempt to successfully clone him. 1157, the one in the tank, is the successful clone.
Or.... the Starkiller you played as for the majority of the first game, after Vader betrays you, was another clone. That'd be weird.