What is the ruling on someone who caught two rak'ahs of Fajr prayer as a latecomer, then thought he had missed one rak'ah, so he stood to perform it, then remembered that he had caught both rak'ahs, so he completed the rak'ah and performed the prostration of forgetfulness (sujood as-sahw)?

It was obligatory for you to sit as soon as you remembered that you had completed two rak'ahs. As long as you intentionally added to it, your prayer is invalid, and you must repeat it. However, if the addition was due to your ignorance, then your prayer is valid, and you do not have to repeat it, provided that you are among those who are excused for being ignorant of these rulings.