You are taxed only once: with income tax. Company is also taxed only once: with corporate profit tax. The reason why that makes sense is because corporation is treated a separate legal entity (person), and that's a good thing for you, because you are not responsible for company's mistakes (that's why they are called "limited liability companies". Publicly traded companies also have limited liability).

Also, if, as a majority shareholder you appoint yourself as the CEO and pay yourself a salary, it is tax deductible for your company, so money that goes to your salary is taxed only once.