Jo Martin's explosive reveal as the Doctor doesn't mean Jodie Whittaker is going anywhere soon, though she can't publicly say how long her contract is for. This isn't the first time multiple Doctors have shared the screen — after all, this is a show about time and space featuring a quasi-immortal alien that can bend those rules as she dares-slash-pleases. Exactly what Martin's iteration of the Doctor means, however, remains mysterious.
Between the fact that Whittaker and Martin's character-that-is-the-same-person don't recognize each other and the fact that Martin's Doctor doesn't know what a Sonic Screwdriver is, Doctor Who fans are largely presuming Martin's version is backdated to the origin of the series – perhaps even before William Hartnell's time. (The Sonic Screwdriver wasn't introduced until the Second Doctor, Patrick Troughton.)
Martin's Doctor could be one or a combination of many concepts. We have previous examples of Doctors physically splitting themselves (David Tennant's Ten-Too), Doctors erased from time and even their own memory (John Hurt's War Doctor), and people who have accidentally gained the powers of a Doctor and lost their own identity (Donna's freak accident that led to her departure as a Companion). However, she isn't from an alternate universe, according to showrunner Chris Chibnall.
Whatever the reason, it almost certainly ties back to the overarching mystery of Whittaker's tenure surrounding The Timeless Child — a secret so terrible, the Gallifreyan hierarchy deleted it from all Time Lords' recollection and The Master leveled his own home planet in vicious retribution when he learned its truth. Doctor Who seeks to strike at the heart of everything the Doctor has ever believed about herself and threatens to turn it all upside-down. Jo Martin won't be a one-off, and will in every way make her own indelible mark standing up with the other 14 people who hold the lofty title of the Doctor.
