About the only thing more baffling than Simon agreeing to let Chicago’s Derek Dupree take a one-hour break and re-audition on last night’s season premiere of American Idol was the woeful warbler’s choice of material for his return engagement. So where exactly did the dude dig up that song with lyrics about ”Muffy in the bathroom stall”? Perhaps he’d recently rented the 1983 Eddie Murphy-Dan Aykroyd comedy Trading Places; my sharp-eyed editor Tom found a corroborating clip from the movie that may explain it all. Maybe Fox should play it back for the whopping 35.4 million viewers (a season-premiere record) left scratching their heads Tuesday night, eh?