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By Paul Lienert and Joseph White DETROIT (Reuters) - Detroit's automakers, on track for their best sales year since 2006, may want to brace themselves for rockier times ahead. Auto executives say the industry is as healthy as it's been since being restructured in 2009. After reporting healthy April U.S. car sales on Friday, stocks fell again at all three. U.S. sales growth this year has slowed to 6 percent from double digits in 2010-2012.