Clara: So Osgood's been kidnapped, right? I thought Osgood was dead.
Kate: There have always been two of her, ever since the cease fire. We never knew which one was real.
The Doctor: Both of them.
Kate: Okay, which one was Zygon.
The Doctor: Both of them. They would have maintained a live link. They were both Zygon and human at the same time.