What do I do if my tax return is rejected because someone claimed my defendant unlawfully?

If you or a dependent that you are entitled to claim has already been claimed by another taxpayer, your e-filed return will reject since the 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.

If neither taxpayer amends, the IRS will send a second letter asking for each taxpayers proof that they are entitled to the claim.

The IRS will evaluate each taxpayers claim and award the credit to one taxpayer, the other will have to payback any refund received plus interest and possible penalties.

Do not ignore the letters or you will loose.

You will have to file by printing and mailing your tax return.  You will receive a letter from the IRS (eventually) requesting proof that you can claim your dependent.  As MacUser_22 said, do not ignore any correspondence from the IRS!