Salat al-Jumu'ah is a prayer, where all Muslims of a particular area, upon whom the prayer is obligatory, gather in one place to listen to the Khutbah sermon and to perform the prayer in congregation, displaying unity which is a symbol of Islam.
Generally, in Islam, there are strict conditions and guidelines to commence a Jumu’ah in one area, where Jumu'ah is already being conducted. The Majority of the scholars, including those of the Shafie Mazhab, have stated that if a second Jumu’ah is conducted without a valid excuse, within a particular area where Jumu'ah is already being conducted, the second Jumu’ah congregation shall be invalid.
Moreover, conducting two or more consecutive Jumu’ahs in one mosque is against the teachings of Islam. This method was not in practice, neither at the time of the Prophet sallallahu 'alayhi wasallam, nor the Sahabah, nor in the era of Taabi’een and the generation that followed them.
The likes of Imam Subuki, the most prominent scholar of the Shafi'i school of thought, have mentioned that there is a consensus (Ijma’) of Scholars that conducting two Jumu'ahs in one Masjid is not permissible.
According to verse 09:62 of the Holy Qur’an, engaging in business activities at the time of Jumu'ah is prohibited. If two Jumu'ahs are conducted in one mosque, the public would be led to a confusion as to when they should abstain from engaging in business activities.
Many Fatwa organizations such as Darul Uloom Deoband, Permanent Committee for Scholarly Research and Ifta of KSA and many prominent scholars have given the ruling that it is impermissible.
In Sri Lanka, too, the same system has been in practice for decades. Few Masjids have sought permission from the All Ceylon Jamiyyathul Ulama, highlighting the need to start two Jumuah’s in their Masjid. However, they were informed of the stance of ACJU that it is impermissible to conduct more than one Jumu'ah in the same Masjid, based on the above mentioned evidences. The Jumu'ahs conducted in such a manner will be considered invalid. Accordingly, the people who prayed Jumu'ah in the second congregation, have to repeat the Dhuhr prayer.
If there is not enough space for everyone to gather in one Masjid for Jumu'ah, arrangements could be made to start Jumu'ah in other Masjids; or the devotees can go to a nearby spacious Masjid, where Jumuah is conducted.
Allah knows the best.