If you used the 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-reject-r0000-902-01/01/1602609