If someone missed one rak'ah of Maghrib prayer with the Imam, and then stood up to complete it during the Imam's final tashahhud out of ignorance, and then repeated the Maghrib prayer, must he repeat the prayer again?

The questioner's action is wrong, and it is not permissible for someone who has joined the prayer late (masbuq) to leave the Imam before the Imam completes his prayer. If she did so without the intention of leaving, due to an excuse, her prayer would be invalidated. However, since she repeated the prayer, her obligation has been fulfilled.