I had this problem before when I was convinced that actor Patrick Wymark was the same person as the actor Jay Garner. Actually I still have trouble keeping them apart. And their voices don't help because they even sound more alike than they look alike.
Now that you can see them side by side you can also see why the faces might be mistaken for each other if you only focus on the fact that some features are identical. Look at the eyebrows and nose of the two actors in the lower image, virtually identical. Now with older men it's always trickier because they tend to look more alike, but still.