Consumer.poll() returns new records even without committing offsets?

The starting offset of a poll is not decided by the broker but by the consumer. The consumer tracks the last received offset and asks for the following bunch of messages during the next poll.

Offset commits come into play when a consumer stops or fails and another instance that is not aware of the last consumed offset picks up consumption of a partition.

KafkaConsumer has pretty extensive Javadoc that is well worth a read.

Consumer will read from last commit offset if it get re balanced (means if any consumer leave the group or new consumer added) so handling de-duplication does not come straight forward in kafka so you have to store the last process offset in external store and when rebalance happens or app restart you should seek to that offset and start processing or you should check against some unique key in message against DB to find is dublicate