@Override
      public void goingToFlush() {
        if (!alreadyComeHere) {
          // this should do an in place update of the re we just took of the queue
          region.put("Key", "Value2");
        }
        alreadyComeHere = true;
      }
A put is done on the same entry before flushing that entry

The test ensures that put is not done twice by using a alreadyComeHere boolean.
geode-core/src/integrationTest/java/org/apache/geode/internal/cache/ConcurrentFlushingAndRegionOperationsJUnitTest.java