What is the expiation for not performing the 'Isha prayer for several months, and is it permissible to perform the 'Isha prayer twice daily to make up for the missed ones?

Prayer is a great pillar after the Two Testimonies, and abandoning it is an act of disbelief (kufr). The lazy one who abandons prayer is asked to repent, otherwise he is killed. The lazy one who abandons prayer is considered an apostate (kafir) by the Hanbalis, and he must renew his faith and is not required to make up for missed prayers. However, the majority of scholars believe that he is a Muslim who has committed a grave sin, and he must repent and make up for the prayers he missed.

To make up for missed prayers, one must know their number. If you do not know, then make up what you believe will clear your conscience. Making up for missed prayers involves performing them immediately, while observing their proper sequence. This act takes precedence over all other deeds, except for the five current obligatory prayers.