Ah, yes, Super Bowl weekend. Football on the wide screen. Homemade chili simmering on the stove. And, of course, cold beer in the fridge. Walk into almost any supermarket or liquor store these days and you’ll find an abundant assortment of beer — the big domestic brands, along with imports from every corner of the globe and craft brands that are proliferating at the rate of one per day. In terms of quality and choice, this looks to be the golden age of beer drinking in America. But beneath this bewildering variety lurks a very different market reality. An emerging duopoly has come to dominate the brewing, marketing and distribution of beer no less than Coke and Pepsi have done for soft drinks.