Twin

Romulus and Remus Sylvest were identical twin brothers. (TV: The Twin Dilemma)

As Donna Noble recalled, Nerys fathered twins with a turkey baster. (TV: The Doctor's Daughter)

Gwen Cooper observed that Jack Harkness and Ianto Jones talked like twins. (TV: Children of Earth: Day One)

Amy Pond likened the Gangers and their human counterparts to identical twins. (TV: The Rebel Flesh)

Elizabeth I refered to a Zygon whom assumed her form as her twin. (TV: The Day of the Doctor)

When Tyler Steele met Jack Harkness again at 3Sol after seeing him die at the candlelight vigil outside of the Refugee Crisis Centre, he asked if he was a twin. (AUDIO: Changes Everything)