You should file a correct return. If you are the parent with custody more than half the nights of the year, and you did not give away the child tax credit, then print your return as-is and file by mail. The IRS will pay your refund and investigate the duplicate claim later.
If you did allow your ex to claim your child, you need to re-run the dependent interview to say "yes, the child lived with me more than half the year" and "yes, I gave the other parent a form 8332 to release the dependent credit." However, even then, you might be blocked from e-filing if your ex answered these questions wrong on their tax return, and you will still need to print, sign and mail your return.
