Talk to a local CPA or tax professional to get your question answered OR call the IRS toll-free number to ask them. You may not have to do anything. If you claim the child and your ex also claims the child, the error is going to be caught by the IRS and they will be contacting either you or her to get an explanation. YOU will have valid explanation and a valid claim, and SHE won't--they will disallow her deduction and maybe even penalize her.