Is one expiatory offering (kaffarat yamin) sufficient to be released from a "nadhru al-lajaj" (an oath made in anger or to prevent oneself from doing something), or is an expiatory offering required each time the condition of the vow is met?

What you mentioned, that the vow of Lajaj (a vow made in anger or argument) gives a choice between fulfilling it and offering an expiation for an oath (kaffarat yamin), is the most preponderant opinion. A single expiation for an oath is sufficient to dissolve the vow. So, if he does what he vowed to abandon, or abandons what he vowed to do, after offering the expiation, nothing further is binding upon him. However, if the wording of his vow implies repetition, such as if he said, "Every time I do such-and-such, then such-and-such is binding upon me," then the expiation becomes binding upon him with each repeated occurrence of the action he vowed to abandon.