Is one considered sinful if they do not repeat the Maghrib prayer after hearing another caller announce the prayer time after they have already performed it? And what is the expiation for that if there is sin?

The time for Maghrib prayer is known by two methods: either by verifying the sunset through direct observation, or by emulating a trustworthy person knowledgeable of prayer times. If the first Adhan (call to prayer) was made by a reliable person who knew the time, then your prayers are valid. However, if it becomes clear that the second Adhan coincided with the entry of the time, and the first was before it, then the prayer is invalid and must be repeated.