A man performs Maghrib behind an imam who is praying 'Isha and completes the prayer with the imam by mistake, then repeats Maghrib and 'Isha; what is the ruling on his prayer?

The questioner : May Allah reward you, O Shaykh. I was traveling and found a person in the restroom. I agreed with him and said, "Did you pray Maghrib and 'Isha?" He said, "No, and we are between Maghrib and 'Isha." When we entered the mosque, we found a person praying, and we did not know what he was praying. We entered with him because we wanted to pray Maghrib. He was praying 'Isha, four rak'ahs. We prayed with him, and we prayed with him from the first rak'ah. We did not sit in the third, or perhaps I sat but I doubted. When the person next to me, who entered with me, stood up with the imam for the fourth, I doubted that the imam was praying Maghrib. I doubted, meaning I was distracted and did not notice. I thought that I was supposed to sit in the third. In any case, we prayed with him four rak'ahs. When I finished the prayer, I asked the person next to me who entered with me, and he said that he prayed four rak'ahs. What is the ruling?
We exerted our effort and repeated the Maghrib prayer three times, then we prayed 'Isha two times after that. What is the ruling? May Allah reward you.

The Shaykh : There is nothing wrong with this; what you did is valid. However, if you had intended Maghrib and then stood up for the fourth, you should have sat, recited the tashahhud, and said the taslim, then joined him in the remainder of the 'Isha prayer and completed it. This is the method for this issue. But you repeated it. May Allah reward you with good.

The questioner : May Allah reward you, O Shaykh.

The Shaykh : Yes?