Retirement tax questions

Sorry just to make sure!  The money I’m taking out to contribute to the SEP IRA this year is money I previously earned 5+ years ago that I had paid ordinary income taxes on. So if I’ve already paid taxes, and then I will pay taxes in the future on the same money, wouldn’t that be double taxation?  

 

Or is it that I’m swapping my old taxed money for non taxed money because of the deduction I can take this year?