Brat Farrar (1986)

When Alex Loding passes Brat Farrar in the street, he's struck by Brat's uncanny resemblance to Simon Ashby. Simon had a older twin brother, Patrick, who disappeared a decade ago, and was presumed drowned. Simon now owns the family farm, Latchetts, a top-notch stable. Alex asks Brat, who loves horses, if he'd care to impersonate Patrick to disinherit Simon. But as Brat grows closer to the Ashby family, he finds the role a dangerous and wearing one; especially as he begins to suspect that Patrick's death wasn't an accident.