Is expiation (kaffarah) for an oath (yamin) obligatory every time a person does something that he had forbidden upon himself?

Scholars have differed regarding the implications of a prohibition [tahrim], and the relied-upon opinion is that it necessitates an expiation for a broken oath [kaffarat yameen]. Only one expiation is required, unless a صيغة [phrase/formulation] indicating repetition is used, such as "كلما" (every time/whenever).