While a child can only be claimed on one return per year, that has nothing to do with taxpayers getting the advance payment incorrectly, because they never provided a correct status at the CTC portal. Or, things changed for them. That always an issue with expecting taxpayers to follow some plan, and for trying to give out money in advance of finding out who really is eligible for it.

There is a provision to attempt to claw back CTC overpayments, but it's a very high bar. For this and the EIP, it appears the US taxpayer is out a lot of money that didn't go how Congress intended. It's not the first time and it won't be the last.