You will have to print and mail your return. Then the IRS should send you each a letter to determine who can claim that ssn.
How to mail a return from the Online version
Be sure to attach copies of your W2s and any 1099s that have withholding on them. You have to mail federal and state in separate envelopes because they go to different places. Get a tracking number from the post office when you mail them for proof of filing.
I'm a non filers so what else can I do
If you used the IRS non-filers website for the stimulus payment then you filed a 2019 federal tax return. The non-filers website is only if you had no requirement to file a 2019 tax return.
Now that you are filing a 2019 tax return the IRS is rejecting your e-filed return since the IRS has already received a 2019 return with your name and Social Security number.
See this TurboTax support FAQ for how to proceed in filing the 2019 federal tax return - https://ttlc.intuit.com/community/tax-topics/help/stimulus-how-to-file-your-return-after-getting-rej...
generally (before COVID-19) if a a return not your own was filed with your SSN, you can't e-File and must file on paper via US Mail.
And yes it was not your own, it was filed by the IRS.