With her pooch in tow, the Gossip Girl star celebrates her 21st b-day on the New York City set of her show. Plus: Ashlee Simpson, Lindsay Lohan, Paris Hilton and more

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IT'S HER BIRTHDAY!

Blake Lively celebrates her 21st birthday Monday by reporting for duty with her cuddly pooch on the New York City set of Gossip Girl, where she was serenaded with "Happy Birthday" by waiting paparazzi.

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