It is a loophole in the way Congress wrote the law. The 3rd EIP is really an advance on a credit for a 2021 tax return, but because they wanted those payments to go out in a hurry, they based many of them on 2020 returns. If a parent who claimed the child in 2020 got the payment they do not have to pay it back. But the person who can claim the child for 2021 can get the payment as a recovery rebate credit on their 2021 tax return.
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