Capital gains are not really double taxation. Sure he paid tax, but then he earned more. Many countries charge income and capital gains at exactly the same rate, eg the UK does now.
And where I live, I pay state income tax, buy a car and buy sales tax, and then every year after that I pay property tax on the value of the car. It's a triple tax!
I'm not really sure why capital gains is such a problem. And truthfully a lot of capital gains (maybe even most?) are not doubled taxed. If you buy stock with after-income-tax money, then yes you pay that tax twice. But if the capital gain is derived from equity compensation a lot of times you don't pay income tax so it is only a single tax. That is what the Buffett rule is all about (and this article as well).