What should I do if it says that a tax return has already been filed under my SS number?

[xmasbaby0]

If you used the IRS non-filer site to get a stimulus check, you will now get a  “duplicate SSN” rejection.  We are seeing people who did not notice or ignored the warning on that site about using it if they would be filing a tax return.

https://ttlc.intuit.com/community/tax-topics/help/stimulus-how-to-file-your-return-after-getting-rej...

[Texas Roger]

This has happened to a lot of filers who used the non filer portal to register for the stimulus payment. The portal efiled a tax return with your SSN and $0 income. As a result you cannot efile your return.

See this for what to do: https://ttlc.intuit.com/community/tax-topics/help/stimulus-how-to-file-your-return-after-getting-rej...