Just to let you know how easy this mistake can happen here is a true story ... the IRS only looks at the first 4 letters of your last name, year of birth and the SS# ... my client gave me the wrong SS# (off by one digit)for his child and the dependent was rejected (already filed)  ... we fixed the incorrect SS# and they were accepted ... but this means that someone with the same first 4 letters of their last name,  year of birth and the SS# (that was off by only one digit) filed before my client ... if we had filed first with the wrong # then that other person would have gotten the "already filed" notice ... so innocent mistakes can happen ....

Start by double-checking your Social Security number in TurboTax. If it's wrong, correct it and then resubmit your return. If you entered it correctly then...

Next check to see if you accidentally filed yourself by mistake earlier:

Check Your E-File Status on this Secure TurboTax website
https://turbotax.intuit.com/efile/efile_status_lookup.jsp
- It will show the date you transmitted your return and if it was accepted or rejected.  
- Both your Federal and State will be listed separately if you filed both.
- It will tell you when you E-filed
- The electronic filing date

Then if it wasn't you: 

Then it's likely that another taxpayer, inadvertently or otherwise, filed their return using your Social Security number.

You won't be able to fix this rejection; you'll need to file a paper return and attach a copy of the IRS form 14039 http://www.irs.gov/pub/irs-pdf/f14039.pdf

You "may" be the victim of tax-related identity theft.  Please see this TurboTax FAQ for information on how to proceed:

https://ttlc.intuit.com/questions/1900655-tax-related-identity-theft

We strongly recommend that you call the IRS Identify Theft Hotline at 1-800-908-4490. You can also click here for additional information at IRS.gov.  http://www.irs.gov/Individuals/Indications-your-identity-may-have-been-stolen-and-how-to-report-it-t...