Filed Extension, received "TIN in the Return Header must not be the same as a TIN of a previously accepted electronic return for the return type and tax period indicated"

It sounds like there was already a return filed with your TIN for this year, or the IRS thinks there was. Unfortunately you probably have to call the IRS if you're sure the TIN is correct on the return. 

Thanks for the reply. It seems strange that they would say that as I have not filed for 2021 tax year yet. That is what I am wanting an extension for.... Assuming the TIN is our ss#, they are correct on the form. Could it be that there was an extension already filed for the 2019 (file in 2020) tax season? I have to do both this year. Once again, I really appreciate the assistance.

I would definitely call the IRS because it could be that someone else used your social to file for 2021 either by mistake or for fraudulent reasons. The phone lines will be crowded today too. If you have access to your IRS online account or have not set one up yet then I'd try to go in there too. It will show you all the filings made under your TIN. Here is the link to login or signup:

https://www.irs.gov/payments/your-online-account

Thank you again I will definitely check into that. I believe I do have an online account set up. I appreciate you.