Duopoly or Dual Monopoly is a situation wherein; two companies dominate the market . In other words two companies control production and supply of a product. This might be due to various reasons like example exclusive technical knowledge or know how between the two companies. Two companies hence share the major market share and rivalries become apparent when such companies go in for advertisement or improvement in the product. One major advantage for the consumer is that he gets better quality as both companies rival each other to improve the technical know how. The negative side is that, the consumers wont have a wide variety of choice and have to choice between the two, especially if it is a basic amenity. Also the consumers are caught in the price war between the two.
As a classic example I can think of two companies who fit the bill; who else Coca cola and Pepsi. Coming to IT side can you think of the major companies, it is none other than Microsoft and Open source. In fact when one makes a search for Dual Monopoly, only the case of Open source and Microsoft crops up, so much so for modern technical informations! Though Microsoft has been frowned upon and even sued for their monopoly existence duopoly cannot be dismissed between the two, because of the plethora of technical know how and research that these big wigs are into. One advantage is that they have competed with each other in such a fierce manner that the outcome has been an array of techniques and OS that has made life much more easier. So as an observer one need to watch the outcome and also look out of other smaller industries who are trying to obtain the big slot.