Two Non- Custodial Parents Claim on the Same Child for a Tax Return

I have had my daughter in my care for more then half the year but I was not working so I asked my stepfather to file my daughter as a dependent on last years taxes.he filed and was granted the tax return. Yesterday he received a letter from the IRS with information that two people have filed the same child as a dependent. I know it was the other grandmother and I specifically talked with her to inform her what we where doing for that years income tax return. She agreed and said she would not file my daughter as her dependent. She has admitted to my girlfriend that she did file for last years income tax return as the non custodial parent with out our consent.I would like some advice on how to proceed with rectifying this situation. Thank you!!

Two Non- Custodial Parents Claim on the Same Child for a Tax Return