Yum! Brands (KFC, Pizza Hut, Taco Bell, etc.) is slightly larger than Restaurant Brands International (Burger King). McDonald's is larger than both of them put together. Or if you want brands rather than corporations, there are a zillion burger chains.

> CVS vs Walgreens

Rite Aid and Walmart, among others.

> AT&T vs Verizon

The US wireless carrier with the largest market cap is T-Mobile, with only slightly fewer users than Verizon. There are also many smaller ones.

> Democrats vs Republicans

This isn't a result of consumer preferences, it's a result of the first past the post voting system creating a powerful incentive for the most similar parties to merge until there are only two left. (Score voting fixes this.)

> Apple vs Google

This is the only real market duopoly here, and it's the one they have to worry about.

Either they can license their content to multiple services so it's convenient for customers to subscribe to one service and get everything, or those two companies will eventually figure out a way to get the same effect and most likely end up sucking in 30% of their industry's revenue in the process.

That's how it used to work. They all started making their own platforms and pulling content off of Netflix because they thought they could get the same numbers as Netflix and keep all the payments instead of whatever slice Netflix was giving them.