In the mosque of the locality where imam and muezzin are fixed it is unlawful and makrooh tahrimi to offer salah with jama’ah after the people of the mosque have offered salah in the mosque with azan and iqamah. It is not correct to join such jama’ah. Those who join it do not get the virtues of jama’ah rather due to committing makrooh tahrimi act they become sinful. (For details see: Ma’arif al-Sunan, 2:285-291, Al-Qutoof al-Daniya Fi Tahqiq al-Jamah al-Saniya Dar Talifat Rashidia, p 730-751, Imdad al-Ahkam 2:111, 132-136, 155, Tohfah al-Almai 1:545, 546, and Masail Imamat Mudallal by Hadhrat Mufti Habibur Rahman Khairabadi p 55-59)
Yes, in the portion of mosque which is outside the Shariah mosque, if some people whose salah was missed by chance make second jama’ah then it is allowable. And Shariah evidences are the authority, not the sayings or practice of the people of any area especially when it is against Shariah.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband