On the Double (1961)

American GI Ernie Williams (Danny Kaye), admittedly weak-kneed, has an uncanny resemblance to British Colonel MacKenzie (Danny Kaye). Williams, also a master of imitation and disguise, is asked to impersonate the colonel, ostensibly to allow the colonel to make a secret trip east. What Williams is not told is that the colonel has recently been the target of assassins. After the colonel's plane goes down, the plan changes and Williams maintains the disguise to confuse the Germans about D-Day.