Must I repeat a congregational prayer if I prayed individually and then someone who has not prayed arrives?

You are not required to repeat the prayer with the person mentioned, but it is better to repeat it with him to attain the reward of congregational prayer. It is desirable to repeat the prayer with someone who wishes to perform it, even if you have already prayed. If you repeat it as a follower (ma'mum), it is valid without dispute. However, if you are the imam, that falls under the category of a person performing a fard (obligatory) prayer following a person performing a nafl (supererogatory) prayer, which is permissible according to some scholars. Nevertheless, repeating the Maghrib prayer is a matter of dispute.