First, double-check your entry for the child's Social Security number to be sure it matches their Social Security card exactly. If you are sure it is correct, what you can do is paper-file your return. You (and your child) may be the victim of identity theft.
What will happen is that the IRS will process the return. Both parties that claim the child will receive a letter telling you what to do next. One of the parties will need to amend their return to remove the child. If that doesn't happen, an audit will take place to determine who gets to claim the child. Here is some more information on the process: What to do when someone fraudulently claims your dependent
