It is not taxable and you aren't paying it back. Your 2021 return starts out by assuming you didn't get any Stimulus payments so it gives you credit for the full amount and your refund was too high. Then near the end you enter how much you actually got so it only gives you the difference if any. So you don't get it again.
Look at your return. The Stimulus payments do not show up anywhere as taxable income so you are not being taxed on it. And they are not being subtracted anywhere on your return either.
If you already got the the 3rd stimulus payment last year then line 30 should be zero. Line 30 is only if you didn't get the full amount or qualify for more. So if it was on line 30 the IRS took it off. You tried to claim it twice. There is nothing you can fix or do. Just take your tax return and cross off line 30. Then refigure the math to the end of the return.
