you may not be able to e-file if the person claiming the child files before you. then you'll have to mail in your return. use a method where you get proof of delivery. if you haven't claimed your child for 17 18 and 19 because of this issue you can file amended returns. submit form 14039 Identity theft with your 2020 return.
what will likely happen is both you and the other party will get a notice asking each to amend your return. if neither does, then the IRS sends out a second notice to each asking for proof that you are entitled to claim, proof would include school and medical records, affidavits from neighbors, and even a picture of the child's room with the child. based on this the IRS makes a decision, the winner gets to claim the child the loser gets a bill.
