Does anyone else claim you as a dependent? Are you filing Single, Joint or MFS? Do you have a spouse or children? A ssn on your return has already been filed. It could be just a mistake on someone else’s return. Double check all the ssn on your return. If you can’t get it to go through you will have to print and mail your return and let the IRS sort it out. They may send you both letters to see who can claim that ssn.
