What is the ruling on re-praying the 'Isha (night) prayer in the mosque after the Imam has already led it as a combined prayer (jama' taqdim), and what is the ruling on the prayer of one who caught the Maghrib (sunset) prayer but did not catch the 'Isha prayer with the Imam, then prayed it later with another congregation?

Firstly: If the Imam combines Maghrib and Isha prayers due to rain, and some people are left behind, it is permissible for them to pray Isha in congregation. They are not required to raise the Adhan, but the Iqamah suffices for them, as Ibn Qudamah stated: "If he wishes, he may pray without Adhan or Iqamah; for Urwah said: 'If you arrive at a mosque where people have already prayed, their Adhan and Iqamah are sufficient for those who come after them.'"

Secondly: Combining two prayers due to rain is a concession (rukhsah), and it is not appropriate for a worshiper (ma'mum) to stay behind an Imam who combines, especially if doing so would cause discord (fitnah). Sheikh Ibn 'Uthaymeen was asked about a worshiper staying behind an Imam who combined prayers during light rain, and he replied that the worshiper is not obliged to intend combining the prayers, and that the decisive factor is the presence of the reason for combining. It is appropriate for the worshiper to combine with the Imam to attain the benefit of congregational prayer, unless he believes that the excuse does not permit combining. In that case, he should pray in another mosque that does not combine. If he fears discord, he should pray with them, intending it as a supererogatory (nafilah) prayer, and then pray Isha at its proper time.