To prevent fraudulent returns, the IRS will only accept each dependent's SSN once in their e-file
system.
Because the other parent filed first and used your child’s SSN (even though the other parent did not claim him as a dependent), no further e-files using that SSN will be
accepted.
You will need to print, sign, and mail your tax return to the IRS. Your tax
return will be accepted if you paper file.
If the other parent files first, your tax return will always be rejected because your child's SSN is on the other parent's tax return. The only way to fix this would be for you to file before the other parent.
