Third Doctor (Dead on Arrival)

A version of the Third Doctor existed on an alternate Earth that Jo Grant was once accidentally sent to. After mourning the death of his own version of Jo (her death, unbeknownst to him, being a result of the N-Space Jo's arrival causing her life force to disappear), he returned to UNIT HQ, where he detected a hole in the space time continuum, and somebody on the other side trying to get through, unaware that it was his N-Space counterpart. He dropped dead, when the N-Space Doctor managed to break through to retrieve Jo.

He was mostly identical to his N-Space counterpart apart from, according to his counterpart, being "a bit vainer". (COMIC: Dead on Arrival)