The reject message is saying that the extension request you filed was rejected because another form with the same Social Security Number (TIN) has already been accepted.
If you didn't already file a tax return or another extension form, you might consider mailing another extension form (postmarked today) and getting an IP PIN to use when you are ready to file.
The IP PIN (Identity Protection PIN) is a 6-digit PIN provided by the IRS to certain victims of ID theft or those who wish to take additional steps to protect their identity.
If you don't already have an Identity Protection PIN, the IRS recommends you use the Get An IP PIN tool in order to protect your tax account from tax related identity theft. The IP PIN authenticates you as the valid filer of the return.
See this IRS webpage for additional steps you can take to protect your identity.
See this TurboTax help article for information about using an IP PIN to file your return.
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