When I’m filing my tax return it saying that it is unable because a Social Security number has already been filed. I have not yet filed. Why is this happening?

[xmasbaby0]

The means the IRS has already received a tax return with a SSN that is appearing on your return.   Someone might have accidentally typo'd the wrong SSN or it may have been done intentionally.   Do you have a teenager who had a job and filed a tax return?   Sometimes they answer incorrectly when asked if they can be claimed as a dependent and cause parents' returns to be rejected.

You can print, sign and mail your own tax return, or you can try getting IP PIN's in order to e-file.

GET IP pin in order to e-file if your dependent’s SSN was claimed on another return

https://www.irs.gov/newsroom/irs-takes-steps-to-help-prevent-refund-delays-by-accepting-duplicate-de...

https://www.irs.gov/identity-theft-fraud-scams/get-an-identity-protection-pin

[VolvoGirl]

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.