Michael Phelps

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Michael Phelps competes in the men's 100-meter butterfly on his way to a first-place finish at the Bulldog Grand Slam swim meet at the University of Georgia, Friday, July 11, 2014, in Athens, Ga. (AP Photo/David Goldman)

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Michael Phelps wins the 100-meter butterfly in the at the Arena Grand Prix swim meet in Charlotte, N.C., Friday, May 16, 2014. (AP Photo/Nell Redmond)

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Michael Phelps is interviewed after winning the 100-meter butterfly in the at the Arena Grand Prix swim meet in Charlotte, N.C., Friday, May 16, 2014. Phelps won the race with a time of 52.13. (AP Photo/Nell Redmond)

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Michael Phelps looks at the score board after winning the 100-meter butterfly in the at the Arena Grand Prix swim meet in Charlotte, N.C., Friday, May 16, 2014. Phelps won the race with a time of 52.13. (AP Photo/Nell Redmond)

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Michael Phelps swims on the way to winning the 100-meter butterfly in the Arena Grand Prix swim meet in Charlotte, N.C., Friday, May 16, 2014. (AP Photo/Nell Redmond)

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Michael Phelps stretches before the 100-meter butterfly in the at the Arena Grand Prix swim meet in Charlotte, N.C., Friday, May 16, 2014. Phelps won the race with a time of 52.13. (AP Photo/Nell Redmond)

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Michael Phelps races to a win in the 100-meter butterfly in the Arena Grand Prix swim meet in Charlotte, N.C., Friday, May 16, 2014. (AP Photo/Nell Redmond)

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Michael Phelps dives in at the start of the 100-meter butterfly at the Arena Grand Prix swim meet in Charlotte, N.C., Friday, May 16, 2014. Phelps won the race. (AP Photo/Nell Redmond)

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Michael Phelps looks at the score board after winning the 100-meter butterfly in the at the Arena Grand Prix swim meet in Charlotte, N.C., Friday, May 16, 2014. Phelps won the race with a time of 52.13. (AP Photo/Nell Redmond)