Is it permissible to pray two congregational prayers in the mosque, and is it permissible to pray behind an imam whose condition is unknown and there is doubt about his proficiency in prayer? What should be done if mistakes are discovered in him during the prayer?

If the mosque is one of the commonly frequented mosques (masajid matruqah) or does not have a regular imam (imam ratib), then there is no objection to establishing another congregational prayer after the first one. If it is not one of these types, then there is a difference of opinion among jurists regarding it. As for knowing the imam, it is not a condition for the validity of following him. The basic principle is the validity of a Muslim's prayer and their suitability for imamate unless it is known or highly probable that they violate a condition or a pillar (rukn). In the event of the imam making a mistake in recitation, the type of mistake and its effect must be considered.