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An employee serves a customer at the pharmacy of a Walgreen Co. store in Louisville, Kentucky, U.S., on Monday, Sept. 30, 2013. Walgreen Co., the biggest U.S. drugstore chain, is expected to report fourth-quarter earnings before the opening of U.S financial markets on Oct. 1. Photographer: Luke Sharrett/Bloomberg via Getty Images

An employee serves a customer at the pharmacy of a Walgreen Co. store in Louisville, Kentucky, U.S., on Monday, Sept. 30, 2013. Walgreen Co., the biggest U.S. drugstore chain, is expected to report fourth-quarter earnings before the opening of U.S financial markets on Oct. 1. Photographer: Luke Sharrett/Bloomberg via Getty Images