Start with the beer industry in the US ..basically two major players. InBev- Anaheiser Bush and SA Miller/ Coors Molston
I think you'll find that in most industries, the "natural" progression over time is that they consolidate as they mature until they end up with two dominant players who function as a duopoly. Think of how retail sectors ended up with two dominant competitors, not one ... Wal-Mart and Target, Barnes & Noble and Borders, Home Depot and Lowe's, Staples and Office Depot, etc. Amazon is basically turning the retail industry on its head by adding a new competitor into almost all of these sectors.