Britney performs

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Singer Britney Spears receives cake on her 26th birthday after performing on ABC's 'Good Morning America' in New York. (Reuters/Lucas Jackson)

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Singer Britney Spears performs on ABC's 'Good Morning America' at The Big Apple Circus tent at Lincoln Center in New York on December 2, 2008. (Photo/Bryan Bedder)

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Singer Britney Spears (L) and TV anchor Diane Sawyer appear on ABC's 'Good Morning America' at The Big Apple Circus tent at Lincoln Center in New York on December 2, 2008. (Photo/Bryan Bedder)

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Singer Britney Spears performs on ABC's 'Good Morning America' at The Big Apple Circus tent at Lincoln Center in New York on December 2, 2008. (Photo/Bryan Bedder)

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Singer Britney Spears performs on ABC's 'Good Morning America' in New York on December 2, 2008. (Reuters/Lucas Jackson)

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Singer Britney Spears performs on ABC's 'Good Morning America' in New York on December 2, 2008. (Reuters/Lucas Jackson)

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Singer Britney Spears performs on ABC's 'Good Morning America' show at the Big Apple Circus in New York on December 2, 2008. (AP Photo/Peter Kramer)