Is Moonstruck the only film to win Best Actress and Supporting Actress?

That's a great question! You're right, Moonstruck (1987) is the only film to win Best Actress and Best Supporting Actress at the Academy Awards, with Cher winning for Best Actress and Olympia Dukakis winning for Best Supporting Actress.

However, there have been other movies where two actresses from the same film have been nominated for Best Actress and Best Supporting Actress at the Oscars, but only one film has managed to win both awards in the same year.