Celine:
"Have You Ever Been In Love" - The intro notes sound like some parts from Janet's "Again" and Ginuwine's "Differences"

Mariah:
"Vanishing" - The part that goes "If I Could..." sounds similar to the Jackson 5's (well I guess it's also Mariah's song too, but anyway...) "I'll Be There" with "You and I...". I know it's only a few notes, but with those being the opening lines of both songs, they've always stood out to me...