Duplication/cloning. And I would make it a pool power (hear me out).
We know the capability is available, because we fight our duplicate (with our powers duplicated) in a series of missions in Faultline in the teens/early 20's.
If you make it a Mastermind, it takes up one of the power sets to do it, so a real duplicate with both primary and secondary abilities can't be done, as the cloning process takes up the primary power set.
So, my thought is, make it a pool power. Whereas flight as 5 different levels (hover, fly, group fly, afterburner, air superiority), the pool levels would be number of clones available, and would grow in power with the character .
This process also allows for players to choose the number of clones they want exactly (i.e. - with phantom army, i have to have 3. if i just want a twin, i can buy just the one clone).
And please, in the name of all that is good and fun, please make them costume customizable, so I can have a red, a white, and a blue for Independence Day!