public void register(SystemStreamPartition systemStreamPartition, String offset) {
      registerCount++;
      assertEquals(expectedSystemStreamPartition, systemStreamPartition);
    }
Verify that if a particular key-value is written, then another, then the original again,
that the original occurs last in the set.
samza-core/src/test/java/org/apache/samza/coordinator/stream/TestCoordinatorStreamSystemConsumer.java