Is it obligatory for me to pray with someone who is performing the prayer alone if I have finished my prayer, and what is the validity of the saying, "There are not two witr prayers in one night," regarding witr?

If a person repeats the Maghrib prayer, they should add a fourth rak'ah to it, making it a nafilah (supererogatory prayer) of an even number (shaf'an), because performing nafilah prayers as an odd number (witr) is not legislated except for the Witr prayer itself. This is what Imam Ahmad stipulated and is the view of many scholars. As for not praying with the congregation in the second scenario, that is correct, because repeating a prayer in congregation for someone who has already prayed it in another congregation is not legislated except with a valid legal justification, such as attaining the virtue of congregational prayer for someone who prayed alone.