Can one have 2 clients reading a single topic such that they never receive the same message? If one client dies, the other keeps reading and gets all the messages.
In a word "redundant clients" - not for performance sake but for client failover.
All I have seen is examples of N partitions and >N clients in a consumer group where N clients get messages and the rest are idle. It's not optimal to have 2 clients on a single partition where one client does nothing until the other client fails.
More than one clients in the same consumer group cannot be assigned the same partition at the same time, therefore will never receive the same messages
The scenario you're asking for is more fault tolerance than load balancing... Assuming one partition, if you run two consumers and one encounters some fatal exception while consuming a message and doesn't commit that offset and the client dies, then the secondary idle consumer will pickup from the last committed offset and try consuming those same messages after the consumer group rebalances