What is the ruling on congregational prayer that follows a previous congregation, and what should the worshiper and the imam do in this situation?

It is disliked to hold two congregational prayers in one mosque at the same time intentionally. However, if some worshipers establish a second congregation without knowing about the first, and only realize it after starting their prayer, then there is no blame upon them, and they should complete their prayer. It is better for whoever enters the mosque to join the better of the two congregations.