If you are asking about this loop hole ...
Since the advance stimulus credit was based on either the 2018 or 2019 returns so if you claimed the child on the 2019 return and got the stimulus for the kid you get to keep it.
Then if the child is claimed by the ex on the 2020 return then he gets the credit for the child again and it is legal.
Due to this loop hole in the law they did not change on purpose the stimulus can be claimed on the same child twice if both parents otherwise qualify based off their incomes.
