The word "re-occurring" refers to the repetition of an event or situation. Its antonyms are words that describe the opposite, such as "unique," "rare," "unexpected," and "unscheduled." These words imply that the event or situation in question is not recurring, but instead is a one-time occurrence or an irregular event. For example, a unique event is one that is not expected to happen again, while a rare occurrence happens infrequently. Using antonyms of "re-occurring" can help to convey the idea of something that is out of the ordinary, rather than routine or repetitive.