If I invest $100 into a stock and sell it for $150, you claim that it is double taxation to tax the $50, since tax must have been paid, either on me getting the $100to invest to begin with, or the corporation having paid tax on its profit, leaving the $50 for me.
If I paint houses, and am paid $50 for my work, why isn't that double taxed by the same logic? Either it is double taxed because the guy paying the contractor I work for had to earn the money that leaves me with $50, or, if I am paid directly, the guy who hired me had to earn the $50 to pay me, and he was taxed on his own earnings.
Every dollar is taxed multiple times as it moves through the economy, as long as it translates work into some kind of money. If you want to argue we ought to come up with a system to avoid that, fine, but I don't see that as particularly feasible at the moment.
Point is, distinguishing capital gains as "double taxed" versus income derived from labor is a bogus distinction, and always has been.