Is it obligatory to fast for two consecutive months for each time the sin is repeated, or is it sufficient once for the repetition of the vow for the same sin?

Your vow is called a "vow of obstinacy" (nadhr al-lajaj). After breaking it, you have the choice between fulfilling it (fasting two consecutive months) or offering an expiation for an oath (kaffarat yameen).

If the wording of your oath was: "If I commit a sin, then I shall fast two consecutive months," then the fasting does not repeat with the repetition of the sin, and the vow is dissolved with the first breach.

However, if the wording of your oath necessitated repetition, such as: "Every time I commit that sin, then I must fast two consecutive months," then fasting becomes obligatory every time you commit the sin.

If you choose to offer an expiation for an oath, it will repeat with the repetition of the sin if the wording of your oath necessitated repetition. However, if it did not necessitate repetition, then a single expiation for an oath is sufficient for you, as a vow of obstinacy (nadhr al-lajaj) is treated like an oath.