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.
If you filed a "non-filer" return to get the stimulus faster, you will need to mail your tax return now.
If you did not file a "non-filer" return, you might be a victim of identity theft. You will still need to print, sign and mail your tax return, but you may also want to check your credit report, bank accounts, credit cards and so on for unusual activity.
Don't forget to also print, sign and mail your state tax return, if you have one.