From my experience (which are near 0 for booking mistakes), Carnival does have a window of opportunity to correct mistakes.
One year, my kid's name was spelled wrong. I noticed it upon getting the email confirmation, and reviewing such. This was within minutes after booking, and I quickly called back my TA. TA was able to correct the mistake without any charges. I assume a PVP could too.
Yea, everyone makes mistakes, but IMHO, its the customer who has a responsibility to review their booking, and point out any mistakes quickly (like an unintended double booking of their son).