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.
Your only recourse is to mail file a correct tax return and let the IRS sort it out.
If you did not use the non-filer site, then another possibility is that someone else filed a tax return and claimed you as a dependent. You will not be able to e-file this year. Your return will have to be printed, signed and mailed. The IRS will sort out if you could have been claimed as someone else's dependent.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.