I'm not sure whether this should never be or must be. Sam Becket leaps into The Doctor, and a previous Doctor is drawn to investigate (which doesn't violate laws of time because The Real Doctor isn't really encountering himself as long as he stays away from the waiting room.)
It's called "The One and a Half Doctors."
-- Joe
Hate is the only enemy
It's called "The One and a Half Doctors."
-- Joe
Hate is the only enemy