While "A.I." teased their "Idol Gives Back" charity benefit by revealing that "two of the biggest stars in musical history come together," sources at last week's taping tell TMZ that Celine sang Elvis Presley's hit, "If I Can Dream" -- with an Elvis impersonator! Ugh! But we're told the impersonator was just used as a stand-in, and what you will actually see when the show airs is Celine singing with a virtual Elvis -- in the vein of Natalie Cole and Nat King Cole's "Unforgettable." Unbelievable!