While monopolies tend to get a lot of investor attention (and plenty of regulatory scrutiny), there are some duopolies out there too that might warrant a closer look. If done responsibly, there's nothing wrong with duopolies existing and operating, and investors can make an excellent return for owning one (or both) parts of a duopoly.

Investors right now ought to consider investing in one duopoly in particular: card-issuing leaders Mastercard (NYSE: MA) and Visa (NYSE: V). While there are other credit/debit card issuers, these two firmly control the top two positions. Both stocks have been phenomenal investments over the past decade, with Mastercard returning 671% and Visa returning 503% -- far surpassing the S&P 500's 225% return.

Each company has enviable financial metrics and both reported solid quarterly results recently, but which stock between the two is the best buy for the future?

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Source Fool.com