Does one complete the expiation for the first breaking of an oath, or does one begin a new expiation for the second breaking, knowing that no more than one expiation is required for repeated breaking of an oath concerning a single act?

All praise is due to Allah, who has enabled you to repent from this habit. If you have sworn an oath and broken it more than once, then according to the majority of scholars, you owe an expiation for each oath. However, the Hanbalis hold that a single expiation suffices, but the more cautious approach is to adopt the view of the majority. The expiation for an oath is to feed ten needy persons, or clothe them, or free a slave. One should not resort to fasting except when unable to feed or clothe. Regarding the condition of consecutive fasting, there are two opinions, and the more cautious approach is consecution. If you adopt the Hanbali madhhab and are unable to feed or clothe, then fast three consecutive days. The day you already fasted will not count due to the lack of consecution. If you adopt the view of the majority and those who do not stipulate consecution, then what you have already fasted will suffice, and you can complete it, then fast three days for each remaining oath.