Is it obligatory to repeat the Isha prayer that was combined with Maghrib in congregation, when there is another congregation praying Maghrib in the mosque?

There is no need to repeat the 'Isha prayer as long as it has been performed. It was permissible to follow the congregation that was praying Maghrib and 'Isha with them and complete it without shortening (qasr), because one is following someone who is praying a complete prayer. What occurred resulted in two congregations in one mosque at one time.