Escaping from a Russian prison John Sunlight finds Doc Savage‘s Fortress of Solitude in the Arctic and steals a number of super weapons, this lead to him becoming a major problem for the world and the Man of Bronze.
As stated he’s the only villain to face Doc twice, and makes for such a worthy opponent that it’s cemented him as not only one of Doc Savage’s greatest enemies, but also his arch-nemesis.
Thus he’s a favorite opponent to bring in when it comes to Doc Savage comic books. You just can’t keep a good villain down.