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The reason they are heavily touted is the net profit margin globe bookmaker's favour increases the more selections you add into a multiple opportunity. Say you select any random 5/1 supply. If you bet this as being a single the bookmaker would like a theoretical edge in his or her favor of 15%. Taking two such selections however and betting them from a win double, the bookmakers profit margin rises to around 30% ! Yes your win double create a much bigger win from the same stake however on the long term the bookmaker is eating away your capital for the much faster rate