Is it obligatory to make up the day whose fasting was doubted, along with expiation, if it was from the Ramadan before last?

You are required to make up the fast if you are unsure whether you owe a day from Ramadan, because the default is that it has not been made up. You are not required to offer expiation (kaffarah) if you were ignorant of the impermissibility of delaying the make-up fast until after the following Ramadan, or if you forgot that you owed a day.