Here is an easy way to figure out if something is double taxation: income that counts once for GDP but is taxed more than once. If I make $100,000 in NY as an Illinois resident, and both NY and Illinois tax me on that income without giving me credit for taxes paid in the other jurisdiction, that's double taxation. My income is only counted once in GDP, but is taxed more than once.

Corporate taxes are just the natural consequence of passing money through a distinct legal person (the corporation) in a system that taxes income to legal persons. Say there is a 10% income tax. I make $100,000. I'm taxed 10% on that, leaving $90,000. I give $50,000 to you. You're taxed 10% on that, leaving $45,000. Total income, for the purposes of GDP, is $150,000. Total taxes collected is $15,000, or 10% of GDP. No double taxation!

So why does anyone create corporate entities? To get the benefit of the corporation being a distinct legal person: limited liability. It's symmetric: the corporation is treated as a separate person for liability purposes, and is treated as a separate person for tax purposes.