The Ending

I once read somewhere that the movie was a tribute to the Western genre and was about the Western genre dying out.
With this in mind, Fonda's or Franks going back to a duel with Bronson seems to mean that he now realizes he cannot escape his destiny to be the man in black who is typically killed by the hero in old Westerns. He goes back to die like he is supposed to. Harmonica knows it also. "We must end it the way great Westerns have always ended," he seems to say.