As VolvoGirl mentioned above, there is no way of you knowing or finding out who used your dependent's social security number, (SSN). It could have been intentional, (identity theft) or unintentional, (mistake). I would report this to the IRS at 1 (800) 829-1040.
When you get a rejection like this, the IRS is telling you that you will not be able to efile your return because the SSN in question has already been used. The only option you have at this point is to paper-file your tax return. Just print, sign, date and mail in your 2016 tax return.
