If one promises Allah while repenting that, " I promise Allah I'll not do so and so sin again, please forgive me." Then does it fall into taking oath to Allah or in name of Allah. Do we need to do Kaffarah if we happen to fall into same sin. Or we just need to sincerely repent again? Jazakallah

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The promise is not an oath, and it does not entail expiation on breaking it. However, it is obligatory on a Muslim to avoid sins and stay away from them in any case, especially if he promised Allah to leave them. If breaking a promise made to people is dispraised (blameworthy), then breaking a promise made to Allah the Exalted is blameworthy for a greater reason. The Messenger of Allah said, “There are three signs of a hypocrite: When he speaks, he lies; when he makes a promise, he breaks it; and when he is trusted, he betrays his trust.” [Al-Bukhari and Muslim].

Allah knows best.