Github user srdo commented on the issue:
https://github.com/apache/storm/pull/1919
The spout as is doesn't support true at-most-once. When auto commit is on,
the consumer periodically commits offsets. When auto commit is off and ackers
is 0, the spout periodically commits offsets manually. In either case the spout
could crash and replay tuples that were acked but not committed. If you need
real at-most-once, the spout needs to commit offsets immediately when acked,
and in that case you would also need to disable auto commit.
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