If you filed through TurboTax, check your email. They send a receipt or email when you e-file. First they send an email saying that your return has been sent. Then, 24-48 hours after it is sent they will also email you to let you know if it has been accepted.
If you think you filed already, did you happen to save a copy of the return? Like a pdf or anything?
I'm only asking because if you know what your total refund amount should be (or if you can find out), you could use the "Where's My Refund" tool on the IRS website to check the status of the return. If you did submit it already, it will have a status. If you didn't, then it will tell you that also.
Another idea would be to call the IRS. Personally, I'd save this for last because in my opinion it's quite the hassle.
Edit: Otherwise, it would seem someone else has filed in your place. Possibly a spouse or relative, and hopefully not anyone else!
Also, check to be sure your SSN is correct in all fields where it is required. If they don't match, you could possibly receive the same error. And the same thing for any dependents, if you are claiming anyone.
*If you are attempting to claim a dependent, it could be possible that someone else has already claimed them. You could possibly receive duplicate SSN for that reason.
Sorry, trying to cover all bases here.
