What are the consequences for me if I swear an oath not to commit a certain sin, and if I commit it, I will give a sum of money in charity in addition to expiation for breaking an oath, and if the act is repeated, the amount will be doubled along with expiation for breaking an oath?

Abandoning sin is obligatory. If a person swears to abandon it, then abandoning it becomes more emphasized for him, and breaking the oath becomes forbidden. If a person swears not to do something, and if he does it, a charity is incumbent upon him, then if he breaks the oath, he is obligated to offer an expiation for the oath (kaffarat yamin) and fulfill what he vowed in terms of charity or an expiation for the oath in lieu of the vow, because it is a vow of obstinacy (nadhr lajaj). If the sworn-against action is repeated, the vowed charity or another expiation for the oath must be multiplied, along with an additional expiation for the oath for the repetition of the breaking of the oath if it was intentional. So, upon repeating the action sworn against, two expiations for breaking the oath become obligatory, and the fulfillment of both vows or the expiation for each of them.