The phrase "be one case" can be replaced with a number of synonyms depending on the context. Some possible options include "be a single instance," "be a singular example," "be a unique occurrence," or "be an individual occurrence." Each of these synonyms conveys the idea of something being a one-time event or situation, as opposed to a recurring pattern or phenomenon. Depending on the tone or purpose of the writing, different synonyms may be preferable in conveying a specific nuance or emphasis. However, all of these options can effectively convey the meaning of "be one case" in a variety of contexts.