If your dependent has already been claimed and you are entitled to the exemption, you will need to print, sign and mail your return to the IRS for it to be processed. the IRS will eventually contact both of you to figure out who gets to claim the dependent. The losing party will then be liable for any additional taxes, penalties, and/or interest as a result of the false claim.
https://ttlc.intuit.com/replies/3299938
