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He's done so on numerous occasions ("The Five Doctors", "The Three Doctors", "The Two Doctors", "Time Crash", "The Day of the Doctor", and the Eleventh Doctor did it so many times it nearly became "his thing"). Nothing bad seems to happen and they can even shake hand without the energy discharge that usually happens when people encounter themselves. In "Day of the Doctor" it is finally confirmed in canon that the earlier Doctors are unable to remember meeting their later selves after a time. In "Time Crash" it's indicated that the Fifth Doctor, before losing his memories of meeting the Tenth, left himself a signal or a message that restored his memories of resolving that situation.