Since you do not think it is the child's other parent or a family member, this could also be a case of identify theft. You can experiment with removing a child and then e-filing again until you figure out which social security number is being rejected. Then file the return without that dependent.
After your return is processed, you can file an amended return that you will mail to the IRS along with a letter explaining that the child was claimed by someone else. The IRS will then resolve the issue.
Apply for IRS identity protection pins for yourself and all of your dependents. Then no other person will be able to file with those social security numbers.
IRS IP Pin
