I already filed jointly with my spouse, but just got a letter saying I didnt file this year?

[Critter-3]

This sometimes happens when the spouse listed  so respond to the IRS with proof of filing.  A signed copy of the return form 1040 only  with the words "copy only do not process" written like a water mark  and a copy of the acceptance email and/or the filing  history page. 

[xmasbaby0]

A letter from who?   Do you mean you got an email from TurboTax?  Do you have multiple accounts?   If you have an account that you did not finish a return in, that might have generated an email.   If you already filed and your joint return was accepted, then ignore the reminder to file.