By the way, I am not sure what caused the bug. When I first picked Magnus, I was unable to assign him to any city. The "assign" button on the city selection screen was greyed out. So I cancelled and then clicked on the Governor screen again and that is when I noticed I had 2 Magnus governors and I could assign a city to the first Magnus.
No mods. Yes, it is a real bug. Interestingly, the game does not let me assign both at the same time. When I try to assign the second Magnus, it reassigns my first Magnus and shows the second one as still unassigned. So the game is showing 2 Magnus but only letting me assign one. The second Magnus appears to be a graphical "clone".