If I need to mail in my return but the IRS rejects my stimulus form because it says I already filed my taxes (R000-902-01), is that because TurboTax already sent it in?

If you already filed a 2019 or 2018 return, you cannot use the stimulus form.  The IRS stimulus form & TurboTax's Stimulus Registration is only for people who have not filed a return, or do not need to file a return.

Let me clarify: I haven't filed my taxes, I have to mail them in. When I went to the portal to fill out the form in lieu of mailing out my taxes, the form was rejected because the IRS said that my SS number was already used in this year's taxes. This is concerning because, as far as I know, I didn't file my taxes unless TurboTax automatically transmitted them for the stimulus program. Otherwise, I will need to worry about a stolen identity. So does TurboTax automatically transmit your federal taxes for the stimulus program regardless of what you actually need to do to officially file your taxes i.e., mail them in?

TurboTax would not submit your return without your permission. You would have to press the "Submit" button after going through the "File" routine in TurboTax.

 

Are you sure the notice you saw regarding your return being submitted was for 2019?

 

Also, you can get a transcript of your return online from the IRS to see if it has been filed as follows:

 

https://www.irs.gov/individuals/get-transcript

@sarahpayneevans  TurboTax cannot e-file your return.  You would have had to go all the way to the end & click on the Transmit or Submit button.  If the IRS is saying your SSN is on another return then you must have transmitted your return.