Is it permissible to perform a second Jamaat (congregation) in the Masjid?
A group performed Salaah in Jamaat before the Masjid’s timing for Salaah. Can the Imaam now perform a second Jamaat?
In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
With regards to performing a second Jamaah (congregation), in such a Masjid of a locality where there is an appointed Imaam and stipulated timings for Salaah, if the people of that Masjid performed the first jamaah with Athaan, then it is makruh tahrimi (prohibitively disliked) to perform a second Jamaah. However, one may form a second Jamaah out of the actual Masjid area like in the hall area.
This prohibition can be understood from the Ahadith of the Prophet ﷺ
Abu Bakrah (Radhiallahu Anhu) narrates: “Once the Messenger of Allah (ﷺ) came from the outskirts of Madina, and intended to offer Salaah in the Masjid. He noticed that people had already offered their prayer. So he went home and performed his Salat in congregation with his family members.” (Mu’jam al-Kabir of Tabrani and authenticated by al-Haythami in Majma al-Zawaid)
If a second congregation was recommended or permissible, the Messenger of Allah (ﷺ) would have not left the virtues of praying in the Masjid for his house.
2) Also the practice of the Shabah shows that not one of them habitually performed Salaah in a second congregation.
It should be noted that in the situation where some people from the locality came and offered their Salaah in congregation, and other members of the locality were unaware, or the Salaah was offered before the stipulated timings of the Masjid, then it will be totally permissible for others to come and offer their Salaah in a second congregation upon the fixed timings, as this will be regarded as the ‘main’ Jamaah of the Masjid, and this will not fall under the mentioned prohibition.
Therefore the situation in query, the Imaam performing a second congregation at the stipulated time of Jamaah will be permissible and not disliked.
And Allah Ta’āla Knows Best
Mufti Arshad Ali
Darul Iftaa, Jaamia Madinatul Uloom (Trinidad)
 ويکره تکرار الجماعة باذان واقامة فی مسجد محلة لا فی مسجد طريق او مسجد لا امام له ولا موذن در المختار
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قال وإذا دخل القوم مسجدا قد صلى فيه أهله كرهت لهم أن يصلوا جماعة بأذان وإقامة ولكنهم يصلون وحدانا بغير أذان ولا إقامة.. المبسوط للسرخسی1/ 135
This answer was collected from Fatwa-tt.com, which is operated by the Darul Iftaa of Jaamia Madinatul Uloom (Trinidad and Tobago) under the advice and guidance of Mufti Ebrahim Desai (Daamat Barakaatuhum) of South Africa.