The phrase "once and again" can be a tricky one to find antonyms for, as it is somewhat specific in its meaning. However, some good options might include "rarely," "infrequently," or "seldom." These words capture the opposite idea of something happening "once and again," which implies a regular or repeated occurrence. Other options might include "never," "rarely ever," or "only occasionally," as these all convey the sense of something happening rarely or happening only once. Ultimately, the choice of antonym will depend on the context in which the phrase "once and again" is being used, as well as on the level of specificity required.