What is the ruling on someone who prayed Maghrib and 'Isha by Jam' Taqdeem (combining and offering earlier) while traveling, then, upon arrival, found a congregation praying 'Isha and repeated the prayer with them?

If a Muslim prays 'Isha in combination with Maghrib (Jama' Taqdim), and then encounters a congregation praying 'Isha, there is no harm in him praying with them with the intention of a supererogatory prayer (nafilah), because he has already performed the obligatory prayer. It is not valid to perform the same obligatory prayer twice according to most scholars. However, some permit the intention of a fard (obligatory) prayer in a repeat performance, while others leave the matter to Allah Almighty, and still others consider the intention of the second fard prayer as a completion of that for which the prayer was repeated.