Two of a Kind (1951)

Brandy Kirby and crooked Lawyer Vincent Mailer plan to rob William and Maida McIntyre by producing a convincing double for their long-lost son. Brandy charms gambler Lefty Farrell into impersonating the missing son. Kathy, the McIntyre's niece, who likes Lefty, introduces him to the McIntyres who soon become convinced he is their son, but the old man refuses to change his will.