“Paul Simon has brought joy to so many for so long, but on this night he made Rayna Ford’s dream come true. During a show in Toronto on May 7, Rayna Ford, a fan from Newfoundland, called out for Simon to play ‘Duncan,’ and said something to the effect that she learned to play guitar on the song. In a moment of astonishment and disbelief, Paul Simon invited her on stage, handed her a guitar and asked her to play it for the crowd.”
All Songs Considered Blog : NPR
And here is the video: