It might take the IRS 2 months after you mailed in the paper return and Form 866-H-DEP, before they will start to determine who’s entitled to claim the dependent.
I might receive a letter - CP87A see image below - stating your child was claimed on another return. It will explain further actions required by you.
The person who claimed the dependent will receive the same letter.
The IRS also might issue an audit letter, when you will need to provide proof that you’re entitled to claim the dependent. Be sure to reply completely and by the response deadline. After the IRS has decided the issue, either additional taxes, penalties, and interest might be applied on the person who incorrectly claimed the dependent.
See the sample letter or notice CP87A below