Here is the "If I Can Dream" duet Celine Dion "performed" with Elvis Presley on the American Idols show on April 25, 2007. Millions of American viewers watched as host Ryan Seacrest announced "Celine Dion is travelling back to the year she was born, 1968, to sing with the man who is and always will be the worlds Greatest Idol".
Celine Dion was performing next to the Elvis impersonator Ryan Pelton from the Legends show in Las Vegas singing. This is what the live audience saw, for TV she was performing along side Elvis. For the close-ups they used the real Elvis.
Source: Various / Updated: Apr 26, 2007