It can happen that two threads can subscribe to the same topic.
See: https://github.com/camunda/camunda-external-task-worker-java/blob/5c2f69783ba29e909eae06025729d74cd21808ef/src/main/java/org/camunda/bpm/client/topic/impl/TopicSubscriptionBuilderImpl.java#L62-L63
CAM-8804 I can subscribe to a topic with the java external task client
CAM-8808 I can fetch and lock tasks with the java external task client