Ralph's vs. Albert sons (or add your local major grocers). A duopoly is more concerned about competition form without. Compare prices and you'll see that 99 percent are the same.

Political parties cannot compete with the fresh ideas from independents or minor parties because anything new can threaten the existing power bases. Take taxes. There have been several (five?) Presidential Commissions that have examined the IRS and income taxes. Each came to the same conclusion that they are unfair, convoluted and liable to special interest manipulation. Nothing comes of it for the simple reason that the two parties have the same incentive: use the tax system to increase their power. Each candidate can claim he or she is going to raise taxes on the other guy and cut them for you. Bogus, typically, but the power is there.

Either party can stray, as developed at y-2008.com in the media questions, but over the long term, they have to use the duopoly to keep outsiders from stealing the show.