Applications typically choose between at-least-once and exactly-once processing semantics. At-least-once processing guarantees that the application will process all the log records, however when the application resumes after failure, previously processed records may be re-processed if they have not been acknowledged. Exactly once processing is a stricter guarantee where applications must see the effect of processing each record exactly once. Exactly once semantics can be achieved by maintaining reader positions together with the application state and atomically updating both the reader position and the effects of the corresponding log records.
This tutorial shows how to do exact-once processing.