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.
Osgood 1: The Zygons are a peaceful race. Osgood 2: Their shape changing abilities should not be considered a weapon. Osgood 1: It's a survival mechanism. Osgood 2: They embed themselves in other cultures and live out their lives in their new bodies, in peace and harmony. Mainly.