My sister filed her taxes using turbo tax in 2023 and she just got a letter from the IRS saying she file her 2023 taxes?

Sounds like she should check her online account and see what it says.  And she should also check to see if she has proof that she paid 2023 tax due---does she have bank or credit card records to prove the payment of tax due?

When you e-file your federal return you will receive two emails from TurboTax.  The first one will say that your return was submitted.  The second email will tell you if your federal return was accepted or rejected.  If you e-filed a state return, there will be a third email to tell you if the state accepted or rejected your state return.

First, check your e-file status to see if your return was accepted: 

https://turbotax.intuit.com/tax-tools/efile-status-lookup/

If the IRS accepted it, what does it say here?  

https://www.irs.gov/refunds   

  Or does your account say “Ready to Mail?” Or “Rejected?” Or “Printed?”

Note:  If it says “Rejected,” “Ready to Mail” or “Printed” that means YOU have to mail it yourself.  TurboTax does not mail your tax return for you.

If you realize now that your return was not e-filed successfully, then you will need to file it by mail.  E-filing is permanently closed for past year returns.  You will need to print the return, sign and date it in ink, and mail it in.