Shares of Wal-Mart ( WMT ) dipped Wednesday after an analyst downgraded the stock and warned that the company's strategy to crowd out its competitors may be weakening its sales. Goldman Sachs analyst George Strachan lowered the world's largest retailer's rating to in line from outperform, saying in a research note that the company's sales growth may be suffering from "self-cannibalization," opening new stores too close together. The stock was recently down 24 cents, or 0.5%, to $52.26. "If we are correct about self-cannibalization, it would help explain why general merchandise sales are especially weak at Wal-Mart," Strachan said. "Customers are already willing to travel for non-food items. Adding another supercenter several miles from an existing one helps Wal-Mart gain food market share ... however, it probably drives less and less non-food share."