Does anyone remember a short film about a boy chasing his doppelganger?

A short film, from the early 80's I think, about a boy who gets dropped off by his father at the seaside. The boy then see's another boy in the distance and then chases him every where but never quite catches up with him. The movie ends with the first boy seeing the stranger boy getting picked up by his father! Please help me find this...|||Try using the key words in the plot and google search it.