It seems that one is able to have 90% income tax & 90% flat income tax at the same time with yields not impacting each other. Seems kinda strange you can take their income twice.
As I remember the formulas for “tax evasion”, you should get problems after some turns. I did the 90% income tax once, and it turned out no good.
Even with the Tax evasion you still collect way over 100% of wages.
I was thinking about the exact same thing. No one complains if you have income tax alongside flat income tax. Usually in real life you can’t really have a flat income tax with progressive taxation. There are systems where there’s tax progression to some percentage and after that the taxation is flat.
I consider this a bug or huge misbehavior in the game. Moreover it lets you solve all the problems in any nation easily. I played as Finland and Germany and during the first terms I had managed to boost my economy to create tens of billions surplus, pay all the loans and generate reserve of trillions dollars. With such enormous amount of money, I could then afford to pay whatever pleased the people and everyone was happy. Not very realistic I think. By the way I hardly had taxes at all, because during both games my GDP was shooting the stars, so the low taxes were still enough to pay off everything… even the space program for Finland 
Also there was no Tax Evasion either.
Currently the games engine does not permit you to have mutually exclusive rules for policies, so there is no way to prevent the player doing this, although I admit, this is an unfortunate failing of the engine designs and is something I’d like to one day find time to address. This would also allow for a lot more flexibility in other policy areas.
Maybe a “fiscal pressure index”? So the player know that the global taxation imposition is at 49% for example.
is there a laffer curve effect of taxation implemented in the game?