Sergeant Deadhead is a 1965 musical comedy film starring Frankie Avalon. Sergeant Deadhead is bumbling soldier who is accidentally blasted into space. When he returns home he is a national hero but has also developed a massive ego. A soldier who looks exactly like him, Sergeant Donovan, is found to take his place.
Not sure I'll be looking this one up but interestingly it is another film to feature those Men into Space helmets