What is the ruling on someone who caught the Maghrib prayer in the second rak'ah and completed the prayer with the Imam, then stood up to make up the missed rak'ah? Is he required to recite the Tashahhud again, knowing that he recited it with the Imam in the second and third rak'ahs?

The final Tashahhud (definition) in every prayer is obligatory for the person praying alone and for the one praying behind an Imam (whether having missed part of the prayer or not). If someone catches the last two rak'ahs of Maghrib prayer with the Imam, then he must perform the Tashahhud in the first rak'ah of his prayer (to follow the Imam), and in the second rak'ah (to follow the Imam, and this is his second rak'ah, and even if he were praying alone, he would sit for Tashahhud in it). And in the final rak'ah, which the latecomer prays alone, the Tashahhud is obligatory.