The modern high street can give an overwhelming sense of déjà vu. Fans trundling to the football stadium of Tottenham Hotspur, a team from north London, pass six William Hill bookmakers on the main approach. Tourists traipsing along a half-mile stretch of 23rd Street in New York pass five Starbucks outlets. In Tokyo, 7-Eleven boasts 15 stores within a similar distance of Shinjuku station. The crush of chain stores frustrates those who like one-off boutiques. Economists fret for another reason: firms may be cramming markets in order to keep rivals out.