Is one who has covenanted with Allah to abandon a sin, then repeatedly returned to it, considered a breaker of Allah's covenant and a transgressor, and what is the expiation for that?

A servant's return to sin after repenting from it does not harm him if he repents again, for Allah accepts repentance if its conditions are met. The proof for this is the noble Hadith: "A servant committed a sin and said: 'My Lord, I have committed a sin, so forgive me...'" until the saying: "I have forgiven My servant, so let him do what he wills."

Based on this, do not despair. If you return to repentance and become steadfast, you are not among the disobedient, and Allah will erase the trace of your sin from you.

You are required to pay two expiations for breaking your oath twice. The expiation is feeding ten poor people, or clothing them, or freeing a slave. If you are unable, then fasting three consecutive days is more cautious.