On the Double (1961)

American GI Ernie Williams admittedly weakkneed has an uncanny resemblance to British Colonel MacKenzie 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 a target of assassins After the Colonels plane goes down the plan changes and Williams maintains the disguise to confuse the Nazis about DDay