It’s easy to isolate a voice over voices in the background in film. Look on this website and you’ll find the videos with Mercedes talking about the dubbing.
Oh, and the Sound Department that worked on The Exorcist won an Academy Award for Sound (Buzz Knudson, et al.). The sounds you here in The Exorcist are not always what you think they are! 🙂
Did they have to literally go back and dub themselves since Linda’s voice had to be taken out and re-dubbed by Mercedes? That’s like literally having to redo the whole movie after filming concluded.