I tried to e-file my daughters return, but it tells me that it is a duplicate file. However, I did not previous submit a return for her.

Ok  what is the exact error code ?   And  did you claim her as a dependent on your already filed return ?

it says "a tax return with the same social security number has already been submitted". But my daughter is 26...and we did not claim her as a dependent.

it says "duplicate social security number"

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...

I am getting the same error but since i did not create a user account until today, I am not able to check the status of my return using the link below. Is there an alternative way to check status?

I have used the turbo tax CD from costco for many users and never needed to create an account

Also i suspect that this error might be related to my submission to the IRS with bank account information for Covid-19 stimulus payment. It seems that the software was similar to TurboTax and I tried to file my return after getting the stimulus payment. Is there a way to check and correct for that error?

I had this same problem happen. When I called Turbo Tax the reason they gave me was if I had filed by SSN to receive the stimulus check (because my taxes were not yet filed) it will give you this error message. When I checked on my State return I did verify over the phone and they show nothing in their database for a 2019 return.

Hi, I am running into the same issue. System thinks I already filed because I submitted bank info for my stimulus payment. Did you find a solution?

If you submitted the non-filer form, it was processed by the IRS as an original filed return.  Any subsequent filing cannot be filed electronically; it has to be mailed in.

If you file your return with a Form 1040X it will be processed like an amended return. 

However, here are two other options:

1. You don’t need to fill out the 1040X to get your return processed.  All you need to do is indicate in a cover letter to your 1040 that you filed a return to get the stimulus check and that this is a corrected return.
2. You can simply write ‘1040X amended return’ on the top of the paper 1040.

Right now the IRS is not processing paper returns, but there will be no way for your to efile, so this is your only option.  Be prepared for a longer than usual processing time.