I used to claim both my parents as dependent, but this year when I submit my tax return, IRS rejected with the error "Review the Social Security numbers (SSN) for everyone on your return. Make sure they're accurate, and that a dependent's SSN doesn't match the spouse SSN on another return."
My mom has always been my dad's spouse. So is there a new law this year that prevents this?
This did not change this year - it has always been the case that you cannot claim someone as a dependent if they are married and file a joint return. So it is a mystery how you could have claimed them as dependents in previous years, if they had filed a joint return.