The Lawless Rider (1954)

U.S. Marshal Johnny Carpenter turns in his badge in order to return to his ranch where cattle thieves have been operating. Reno Frost is the brains behind the rustling gang, and his main target is the ranch owned by Rose Bascom, the girlfriend of the Marshal. When she discovers that her brother, Jim, is a gang member she sends for Carpenter. He shows up impersonating a gunman named Rod Tatum, his exact double, in order to infiltrate the gang. Then the real gunman turns up, which leads to mass confusion all around.