Doctor Who: Twice Upon A Time

When the Doctor almost regenerated himself, until he met his 12th self, which he was almost regenerated too. A Captain from World War 1, discovers an alien called, Testimony, and he discovers the chamber of the dead, the inside of a spaceship of Testimony. The Captain met the 2 but the same Time Travel Doctors. So, what will happen next, if in the end the Doctor regenerates into a new face?