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The title of Steven Moffat's final episode of "Doctor Who" is "Twice Upon a Time." It aired on December 25, 2017, and served as the 2017 Christmas special. The episode features the Twelfth Doctor (Peter Capaldi) and the First Doctor (David Bradley), along with their respective companions, Bill Potts (Pearl Mackie) and Susan Foreman (Carole Ann Ford). It explores themes of regeneration, identity, and the nature of the Doctor's existence.