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But having got the right titles etc, I was able to dig around the internet, and found 'real' opera singers doing the duet. Gord, what a row, what shrieking, what an assault on the senses!
Callas might have had a loud voice, but darn me it weren't a pretty one.
When Kitty Carlisle and Alan Jones sang it on Night at the Opera, it brought tears to my eyes. With 'real' opera singers, I was diving for the off switch.
Oh well, good to know the title, at least. I'll make a recording off an old copy of the Marx Bros film, and enjoy that.
BB