Ever since Dottie Herman bought Douglas Elliman last spring and merged it with Long Island’s Prudential firm, the competition has been scrambling for its own East End outposts. Elliman’s main competitor, the Corcoran Group, got into the game last month, buying Cook Pony Farm Real Estate. And now another big-city firm is flexing its muscle. Brown Harris Stevens is negotiating to buy fellow Christie’s affiliate Dunemere Associates, a boutique Hamptons firm. And brokers from the Upper East Side to East Hampton are buzzing about the industry’s increasing consolidation and the shaky future of indie brokers like Allan M. Schneider and Tina S. Fredericks. “There’s always room for a small, personalized business,” Fredericks chirps hopefully. “And I’m too old to be nervous.”