How is it legal to tax on taxed income?

How is more than half of your income taxable after receiving income that has already been taxed?

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What income has already been taxed? Like in a IRA? For income like W2 or 1099 if they took out withholding that is not the tax, just an estimated amount towards the actual tax on your income. Are you asking about Social Security or SSDI on a SSA-1099?