I tried to efile (married filing joint). The returns were rejected stating that one of my dependents filed a return claiming themselves. However, going back to the copies that were filed, it clearly says "Someone can claim you as a dependent". Is it possible the IRS entered this incorrectly? I cannot file until I remove the dependent (not sure which one it is as they are both dependents and filed as such.)
There is no way to know or if another taxpayer used your dependents SSN as ID theft or by mistake.
Your only option is to mail file and claim the dependents that you are entitled to claim and let the IRS sort it out.