A first queue stores messages to be consumed. A second queue temporarily stores completely duplicative messages that have message identifiers equal to message identifiers of messages currently being processed s and that have times equal to times of the messages being processed; processing of the completely duplicative messages never occurs. By comparison, a partially duplicative message has its processing delayed until the message of which it is partially duplicative has been finished being processed. A partially duplicative message has a message identifier equal to a message identifier of a message referenced within the non-persistent store and has a time unequal to a time of the message referenced within the non-persistent store is moved from the first to the second queue. A non-persistent store stores references to the messages currently being processed. Once the messages are finished being processed, the references to them are removed from the non-persistent store.