It appears that another tax return has been filed using your dependents SSN, whether by mistake (IRS cross matches the first 4 letters of a last name with the SSN, if someone is one digit off and happens to have a similar last name, their return could be accepted even with the incorrect SSN on it and they would never even realize they made a mistake), or as an attempt at identity theft.
Your action is the same in either case, you must file your return by paper and let IRS sort it out. Be sure when filing by paper, to sign/date the return and attach any W2s or 1099 forms that show federal withholding to the front of the federal return. Your state may have different requirements for filing on paper, the instructions sheet that will be included when you print, should explain what needs to be done.
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