Can I claim my step son who has lived w/us for 6yrs. his mother gave the green light,but then we found out that she is still claiming him.

Someone else claimed my dependent and my return was rejected

If someone else claimed your dependent already, then your e-filed return will be rejected.  Print, sign and mail your own return which claims your dependent.  Mailed returns are processed differently by the IRS.  It will take some time (maybe even up to a year) but eventually the IRS will contact both parties and sort out who could rightfully claim the dependent.  The person who wrongfully claimed the dependent will face repayment of refund $, plus penalties.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.

The IRS looks at who the custodial parent is and who is not.  The custodial parent is who the child lives with for more time during the tax year.

When you allow a non-custodial parent to claim a child, you are only releasing the ability to claim the child for Child Tax Credit under the special rule for divorced and separated parents.

The custodial parent retains tax benefits related to Head of Household filing status, Earned Income Credit, and the Child and Dependent Care Credit.  To be eligible for these credits, the child must have lived with you for more than half the year, so the non-custodial parent is not eligible for those credits.

This article will give you more information: Divorced and Separated Parents