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 tax return. It is not added as income anywhere or subtracted anywhere. Income is only on 1040 lines 1-11. The Recovery rebate credit on line 30 is only if you didn't get the Stimulus payment or qualify for more. So if you already got it then line 30 should be BLANK.
Then if you try to claim it again on line 30 but the IRS already sent it to you they will take it off your return and reduce your refund or send you a bill. Line 30 is only if you didn't get the full amount or qualify for more.
