I won't tell you whodunnit
I saw "The Mousetrap" in London in 1973, and it had already been running for over 20 years. Two of the characters are an older woman and a young woman. When I saw the play, the actress playing the older woman had been in the play in 1953 playing the part of the younger woman. Interesting. And great play. If you get to go, prepare yourself when the lights go out!