What do you really mean?  Was your tax return rejected when you tried to e-file?  Did it say a duplicate return has been filed using your Social Security number?  Did you use the non-filer site to get a stimulus check?  Or were you claimed as a dependent on someone else's tax return?

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...

If someone else claimed you as a dependent, then you cannot e-file this year.  You will have to print, sign and mail your return and let the IRS sort it out.  You have several ways to pay your tax due (which is now late).

If you have federal tax due you can pay by mailing your payment with the 1040V voucher, (which has the address printed on it, having the payment taken out of a designated bank account, or you can pay directly on the IRS website.

https://www.irs.gov/payments/direct-pay

https://ttlc.intuit.com/community/tax-payments/help/how-can-i-pay-my-federal-taxes/00/26212

https://ttlc.intuit.com/community/tax-payments/help/how-do-i-pay-my-irs-tax-due-with-a-check-or-mone...

You must pay your state tax due using the state’s preferred method of receiving payment.  For most states that will be by making a payment to the state’s own tax website, or by mailing a check or money order.

https://ttlc.intuit.com/questions/1900884-what-are-my-options-for-getting-my-state-refund-or-for-pay...

https://ttlc.intuit.com/questions/1901670-how-do-i-contact-my-state-department-of-revenue