My granddaughter lived with me for 10 months in 2021. I provided all cost of expenses. I claimed her on my taxes but my son claimed her first. What should I do?

[xmasbaby0]

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.

[DoninGA]

You will have to print and mail the federal tax return since another tax return has been received by the IRS with child's Social Security number being claimed as a dependent.

You can include the child as your dependent on the mailed tax return and if you are owed a tax refund the IRS will pay the refund.

Within a year the IRS will start an investigation on who was eligible to claim the dependent.  The loser of the investigation has to repay any refund along with paying any penalties and interest.