Miley Cyrus' ex-boyfriend Justin Gaston is guest-appearing on "Idol" this Wednesday ...alongside Season 7 castoff Brooke White.

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The duo will be singing Elvis Presley's "If I Can Dream" ... which is also the name of Gaston's new internet reality show, produced by "Idol" creator Simon Fuller.

Last year's runner-up Adam Lambert will be the episode's guest mentor.