Robot Doctor
The "Robot Dr Who" was a robotic duplicate of the First Doctor (although the actual degree to which it resembled the Doctor has been questioned by some) created by the Daleks on-board their DARDIS, with the mission to "infiltrate and kill" the real Doctor and his companions.
Vicki happened to stow away on-board the DARDIS at the time the Robot Doctor was created, and alerted the real Doctor, Ian and Barbara to its existence. When the whole party met it, the timing of the meeting meant that Ian, Barbara and Vicki couldn't tell which was the real Doctor and which was the Robot, and in the confusion Ian attacked the real Doctor. The Robot kept up the pretence and tried to persuade Ian to kill the real Doctor by smashing his head with a rock; before Ian could do so, the Robot made a fatal mistake and revealed that its knowledge of the Doctor's travelling was outdated by calling Vicki "Susan." Barbara immediately realised that it was in fact the Robot and prevented Ian killing the real Doctor. The Robot was subsequently destroyed in the fight with the Doctor and his companions.
