Brian McKnight/Vanessa Williams
The four finalists were: - Matt Cardle (sang a duet with Rihanna) - Rebecca Ferguson (sang a duet with Christina Augilera) - One Direction (sang a duet with Robbie Williams) - Cher Lloyd (sang a duet with Will.I.Am)
They sang a duet
Run DMC sang Walk This Way with Aerosmith.
He sang a beautiful duet with Siouxie named Interlude. He also sang "I Know It's Gonna Happen Someday" as a duet with David Bowie live a couple times on Bowie's tour.
Eddie Rabbit and Crystal Gale
No, they sang a duet together though.
Kitty White
many times........look on YouTube
Debra Byrd
Carole Ford (?)
I think you're thinking of when can we do this again by the kendalls
"On My Knees" sung with Charlie Rich.