Why is it saying my return has already been filed

Did you use the non-filer site to file for the stimulus check?  If so, the IRS treats that just as if you have e-filed a tax return, and you will not be able to e-file a 2019 tax return now.  Or did someone claim you as a dependent?  If your SSN has been used on a tax return, you cannot e-file.  If your SSN has gone into the system as being used as the taxpayer on a 2019 tax return, the only way you can file now is by mail.

Did you use the non-filers website to register for the stimulus payment?  If so, then you filed a 2019 federal tax return.  

See this TurboTax support FAQ for the procedure to use when the non-filer website was used and now you need to file a 2019 tax return - https://ttlc.intuit.com/community/tax-topics/help/stimulus-how-to-file-your-return-after-getting-reject-r0000-902-01/01/1602609

Or is your 2019 federal tax return being rejected for some other reason?  What does the email you received from TurboTax for the rejection indicate?