Which is better: praying 'Isha in congregation at the beginning of its time or at its end? And is it permissible to pray it twice to attain the reward of the beginning of its time and the virtue of its end?

If you pray 'Isha in congregation at the beginning of its time, it is disliked to repeat it in congregation at the end of its time, due to the prohibition of repeating a fard prayer twice without a reason. However, if there is a reason, such as attending a regular congregation, then the second prayer becomes a supererogatory one.