I Was Monty's Double (aka Hell, Heaven or Hoboken) 1958 Black and White 97 min. (Region 1 - Playable in North America - The US, Canada, Mexico, etc.)
Starring: John Mills, Cecil Parker, M.E. Clifton-James. Screenplay by Bryan Forbes. Directed by John Guillermin.
If official documentation didn't exist, we'd never believe a fantastic yarn this. As D-Day approaches during World War II, British officers Major Harvey (John Mills) and Colonel E.F. Logan (Cecil Parker) plot to give the Germans misinformation about their military strategy. When Harvey spots an actor (M. E. Clifton-James, here playing himself) who looks just like British military leader General Montgomery, he uses him to impersonate the general in North Africa. Clifton-James thus draws German troops away from Normandy, becoming both a hero and a major military target.
It's all true!