Answered by Shaykh Faraz A. Khan
Question: If you make an oath to no longer commit a certain sin, and you later commit that sin, would you have to perform expiation once, or for every time you committed that sin after making the oath?
Answer: Assalamu alaikum wa rahmatullah,
I pray this finds you in the best of health and states.
Expiation would be due only once, as after the first instance of doing the sin the oath would have dissolved. In fact, our Master Qasim ibn Qutlubugha authored an entire treatise on this issue, and he transmits therein consensus of all four imams on the fact that such an oath is dissolved upon the first instance of doing the act. [Ibn Abidin, Radd al-Muhtar]
And Allah knows best.
Checked & Approved by Faraz Rabbani