The proper "behaviour" of the offsets committed back to Kafka / ZK should be "the next offset that consumers should read (in Kafka terms, the 'position')".
This is already fixed for the 0.9 consumer by
FLINK-4618, by incrementing the committed offsets back to Kafka by the 0.9 by 1, so that the internal KafkaConsumer picks up the correct start position when committed offsets are present. This fix was required because the start position from committed offsets was implicitly determined with Kafka 0.9 APIs.
However, since the 0.8 consumer handles offset committing and start position using Flink's own ZookeeperOffsetHandler and not Kafka's high-level APIs, the 0.8 consumer did not require a fix.
I propose to still unify the behaviour of committed offsets across 0.8 and 0.9 to the definition above.
Otherwise, if users in any case first uses the 0.8 consumer to read data and have Flink-committed offsets in ZK, and then uses a high-level 0.8 Kafka consumer to read the same topic in a non-Flink application, the first record will be duplicate (because, like described above, Kafka high-level consumers expect the committed offsets to be "the next record to process" and not "the last processed record").
This requires incrementing the committed ZK offsets in 0.8 to also be incremented by 1, and changing how Flink internal offsets are initialized with accordance to the acquired ZK offsets.