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    Taylor Swift and Carly Simon Duet on "You're So Vain"—Watch Now!

    Carly Simon, Taylor Swift Theo Wargo/TAS/Getty Images for TAS

    As if we needed more mystery surrounding the nameless egomaniac being sung about in "You're So Vain."

    Carly Simon joined Taylor Swift onstage at Gillette Stadium in Foxborough, Mass., on Saturday night to duet on her 1972 hit, an ode to a bad beau that predated the types of relationship-postmortem tunes Swift would one day unleash on her unsuspecting (well, they should suspect now) exes.

    "Every girl out there will be thinking of someone when we're singing," Swift told MTV backstage before showtime, discussing how excited she was to be taking the stage with Simon later.

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    "When I heard 'You're So Vain' I just thought, That's the best song that's ever been written," gushed the 23-year-old pop star. "That is the most direct way anyone has ever addressed a breakup. It's amazing."

    "It's a song that really sort of developed that style of mine," Simon added. "And a lot of it was not knowing that I was developing that style, 'cause a lot of things happen by accident, don't you feel?"

    "Yeah," Swift agreed, "especially with songwriting."

    And the rest has been thinly veiled breakup-song history.

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    "Before I bring out my special guest, I have this question that I always, always had, and I think of you you have probably had this question, too, and wondered what the answer is," Taylor, clad in a black and white-striped top and red high-waisted short-shorts, teased the crowd before introducing Simon. 

    "Who is the song 'You're So Vain' by Carly Simon written about?" she wondered aloud, before tossing her ponytail over her shoulder and saying, "As if."

    Enter Ms. Simon singing with Swift about yachts, an apricot scarf and clouds in her coffee.

    The part of her website dedicated to that particular tune mentions the likes of Warren Beatty (a historically popular guess), Kris Kristofferson, Cat Stevens and Mick Jagger as possible vain types.

    But more importantly, who do you think Taylor was singing about on Saturday night?

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