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?
My guess: For the first version of the scene they’ll have someone saying the other character’s lines the way they should be played so that dialogue looks natural. Once one version has been filmed it’s not hard to play that version’s audio while filming the second version so the actor is actually hearing himself play the other character.