Three of the main characters were the same actor, and yet there are shots with all of them in the same scene. When I saw this as a young adult I didn’t even notice that Mike Myers was playing three roles and was genuinely dumbfounded years later when I found out.
How did they do those scenes so seamlessly?
So, back in the day they used a really simple camera trick called a double exposure.
https://abeautifulmess.com/how-to-shoot-a-double-exposure-in-camera/