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He has gone back on his own timeline (the term used in the show is "crossed his own timeline") several times.

  • The first occasion that we know of was in "The Three Doctors", when the Time Lords themselves arranged for him to do so, because they were under a threat they couldn't otherwise deal with. That involved the First, Second & Third Doctors.
  • The most recent example so far (end of Series 10) was in "Twice Upon a Time", when the First & Twelfth Doctors encountered each other.
  • The most notable instance was in "The Day of the Doctor", when all of the Doctor's first 13 incarnations (the First to Eighth Doctors, the War Doctor & the Ninth to Twelfth Doctors) combined forces to save Gallifrey while also ending the Last Great Time War against the Daleks.
  • In "Father's Day", the Ninth Doctor, with Rose Tyler, crossed his own timeline, so that he & Rose briefly saw their earlier selves.

(There have been other instances, too.)

Crossing his own timeline is something the Doctor shouldn't do but sometimes does do.