If a dependent that you are entitled to claim has already been claimed by another taxpayer, your e-filed return will reject since the child's SSN has already been used (either intentionally or in error such as a mistyped SSN).
Your only recourse is to file a correct tax return, claiming what you are entitled to claim, then print and mail the return.
The IRS will process both returns and pay any refunds. Shortly (within a year) the IRS will mail letters to both taxpayers asking if their tax return was filed in error and suggesting that they amend if they improperly claimed the child.
If neither taxpayer amends, the IRS will send a second letter asking for each taxpayers proof that they are entitled to claim the dependent, such as proof that the child physically lived with them more than half the year. School records, child care records, household receipts, medical bills, etc., that show that the child lives with you should be retained.
The IRS will evaluate each taxpayers claim and award the dependent to one taxpayer, the other will have to payback any refund received plus interest and possible penalties. The parent that had physical custody usually always wins.
Do not ignore the letters or you will loose.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
