The Duopoly of Business

A Duopoly is where a market exists where there are only two sellers. I have concerns about the duopoly's I see developing and the impact this has on other businesses. For example, I see these two in Australia:
* Woolworths & Coles
* Cadbury-Schweppes & CocaCola Amatil

I find myself concerned that these businesses become so powerful, they are able to control the market. Woolworths and Coles can control the price they offer farmers, because where else can the farmer go to sell his produce? Cadbury-Schweppes & CocaCola can squeeze out other competitors because they own the whole manufacturing processes that assist in bottling, warehousing, and finally getting your drink on the shelf. They then can control the price.

What about Fairtrade when it comes to these local issues? What about accountability for these businesses? Should these businesses be more transparent with their practices?