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.
How are they not being taxed twice? A company (which has owners) pays income tax on their profits, so the company and therefore, owners, have less capital remaining. Then, if the owners want to cash out anything from the post-income-taxed margin, they'll have to pay capital gains as well.
It's not that simple. One could just as easily make the case that workers get a lower salary due to higher corporate taxes, therefore they should pay a lower tax rate than owners. In short, it's not at all clear that a higher corporate tax rate is purely a tax on owners, and nothing else.
I pay state income tax on my paycheck, and then I take that money and buy a new computer and have to pay state sales tax too!
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).
We're off the original topic now, but I've never understood why the US doesn't enact a dividend imputation scheme, which nicely solves the double taxation problem on dividend income by giving the recipient of a dividend a credit for the corporate tax paid on that dividend. Australia has such a system.