"Exactly Once" messaging semantics means no duplication or loss of messages.

Exactly-once processing is based on a 'read-process-write pattern', which can be achieved through Transactions, see below links more more information:

1. Exactly once Semantics are Possible: Here’s How Kafka Does it: https://www.confluent.io/blog/exactly-once-semantics-are-possible-heres-how-apache-kafka-does-it/
2. Transactions in Apache Kafka:  https://www.confluent.io/blog/transactions-apache-kafka/
3. Video presentation on the link above, search Exactly one semantics: https://www.confluent.io/resources/ 
4. What does Kafka's exactly-once processing really mean?: https://softwaremill.com/what-kafka-exactly-once-really-means/