Kurt mentions that "Make 'Em Laugh" in "Singin' in the Rain" is a duet. It is, in fact, a solo number performed by Donald O'Connor. The closest to a duet it gets is when he uses a life-size rag doll as a prop. The big Gene Kelly/Donald O'Connor number is "Moses Supposes".
When the girls sing the Tina Turner duet, Chord's bangs are freshly cut far above his forehead. Then in the next scene, his bangs return to their normal long hanging in his eyes look. Obviously this was filmed out of sequence.