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File your return claiming the child you are supposed to claim. You will need to print and mail it in. When the IRS receives your return, they will contact the person who claimed your children for clarification. If you have not released the exemption to that person and you are the custodial parent, they will rule in your favor, and if a refund is due you based on that, then they will issue one to you.
Please see the following answer from SuperUser Hal_Al for more information on how the IRS handles this type of situation:
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