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He can cross his own timestream (travel to a time & place he has already visited), although he rarely does. Crossing one's own timestream is not physically impossible but it's forbidden by Time Lord law (the First Law of Time), as it runs the risk of creating serious paradoxes. However, the first time we saw the Doctor cross his own timestream (in "The Three Doctors"), it was arranged by the Time Lords themselves, because they were facing a threat that was more dangerous than the risks associated with breaking the First Law of Time.

The universe is so large -- in both space & time -- that there's only a tiny chance of the Doctor (or anyone else) crossing his own timestream by accident. When it happens, it's usually because someone or something has intentionally caused it.

  • In "The Three Doctors", the Time Lords did so on the orders of their then Lord President.
  • In "The Five Doctors", Borusa (who was Lord President at that time) used the Time Scoop to bring it about for his own illegal purposes.
  • In "Father's Day", against his better judgement, the Ninth Doctor deliberately crossed his own timestream (& allowed Rose to cross hers) as a favour to Rose. That incident showed some of the dangers of doing such a thing.
  • In "The Big Bang", the Eleventh Doctor deliberately crossed his own timestream in order to give the Auton duplicate of Rory Williams the sonic screwdriver & instructions on how to let the Doctor out of the Pandorica. (That also created a paradox but it was a sustainable one, unlike the paradox in "Father's Day".)
  • In "The Day of the Doctor", "something" allowed the War Doctor, the Tenth Doctor & the Eleventh Doctor -- &, eventually, all the first 13 incarnations of the Doctor -- to get together to save Gallifrey. That "something" also allowed these incarnations (other than the War Doctor, who was already there) to pass through the time lock on the Time War. It was implied, but has never been explicitly confirmed, that this "something" was the Moment.

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