Is only one expiation required for a conditional vow that was not fulfilled, or is expiation required every time the vow is violated?

A covenant with Allah, intended to urge oneself to do or refrain from something, takes the ruling of an oath. If the oath includes the meaning of a vow, then one is obligated to fulfill it.

Whoever makes a covenant with Allah to do something and does not adhere to it, must offer an expiation for breaking an oath, because it is considered a vow concerning something permissible. The expiation for breaking an oath does not repeat if the transgression is repeated.

The expiation for breaking an oath is feeding ten poor people, or clothing them, or freeing a slave. If one cannot find any of these, then fasting for three days.