The concept was to put Celine on stage singing with Elvis as if he was actually there. They filmed faraway shots with an impersonator and the real Elvis ( hologram image ) was overlaid on screen in post production. It was never a duet before april 25,2007. Elvis sang -If I Can Dream - as the final song on his "comeback" 1968 TV special - then broadcasted on NBC-TV april 28,2007