Troughton is the only actor to play his own "evil" twin in the Doctor Who Series. In the episode Enemy of the World he plays both the Doctor and his nemesis, the evil Salamander.
Patrick Troughton: (On playing the Doctor) How did I feel about taking on the role? To begin with, I thought it would last about six weeks after Billy Hartnell had finished. My children and I had been fans of the programme and I loved the way he had played the Doctor. But I knew I couldn't possibly do it like that.