The Doctor Who rumor mill is churning once more, this time propelled by the idea of second companion.

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Despite nearly exiting the series twice, Jenna Louise Coleman will returns as Clara for season 9. As the Impossible Girl traverses the universe with the Twelfth Doctor (Peter Capaldi), however, the TARDIS may also carry another passenger.

As speculation grows, so does the list of available companions. Numerous sites, such as Kasterborous, believe that a new addition to the TARDIS crew would most likely come in the form of a pre-established character, i.e. Missy (Michelle Gomez) or Shona McCullough (Faye Marsay) from "The Last Christmas."

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This wouldn't be the first time the Doctor took on more than one companion. Most recently, Eleven spent several years with both Amy Pond (Karen Gillan) and, later on, Rory Williams (Arthur Darvill). Ten and Eleven both saw Mickey (Noel Clarke) and Jack Harkness (John Barrowman) spend time onboard the TARDIS as well.

Who do you think could become Twelve's second companion, if there is one? Sound off in the poll below!

Find out more when Doctor Who returns to the BBC and BBC America this fall. Click the video below to see a preview for the series' latest episode, "The Last Christmas."