If nothing else, the chase scene at the end of this chapter pretty much confirms that she's not human. Minji's super-punches in the end of the first season were solely by the monster arm, but this "girl" is climbing walls. Also, she was using the box-cutter that the siblings left on the rooftop.
What I don't get is how she's so convinced that she's the real one. I mean, she should have at least her memories of being naked on that rooftop, even though she seems to have duplicated memories of the real Minji. And she should be questioning her own superhuman abilities.
As for the one I assume to be the genuine one, I expect her to be able to climb out of that shaft if she can activate the extra monster arms.
What I find intriguing in this is that both copies share the same goals, so the impostor one has completely reproduced Minji's personality. Which probably means they will cooperate later on in the story.