Hello,
We received the same error. It appears that even though the Netfile screen shows that you can submit both users under one tab it attempts to re-submit the users return that is selected for the interview. I.e. I had just completed my wife's return, submitted hers, and then clicked to submit mine. The screen refreshed to say 'Rejected' with her name and prior acceptance code.
When I clicked on my name in the Interview tab, and then clicked on Netfile to file my return then my return submitted successfully. I assume this is just a bug in the software as we can see the Notice of Assessment in my wife's CRA account, and no mention of errors.
