If the IRS is rejecting your return because the dependent has already been claimed, then his social security number has indeed already been used. They will not give you information on who else claimed that number, so you will have to file on paper in the mail, and they'll send your refund. At some point in the not-too distant future, you'll receive a letter asking for evidence that the child lived with you and other things and whoever has that information will keep the dependent claim.