1968, in Avengers Annual #2. The Avengers of that time battled their alternate selves at the behest of the Scarlet Centurion.
Originally Posted by gurkle
Yes, and I think this was the first Marvel comic (probably not the first comic) to use the story of Alternate Reality versions of the heroes who become authoritarian fascists.
So if this great early example’s right, then the answer to my question might be, The Avengers...
...more specifically, Hank Pym and Janet Van Dyne!