In some very rare cases, Cluster.Connect() throws NoHostAvailableException where every host connection attempt fails with ArgumentException 'The source argument contains duplicate keys.'.
Looking at the stack trace this is coming from the TokenMap constructor when a ConcurrentDictionary is initialized from an existing Dictionary.
When this happens, the only way to resolve the issue is to restart the application.
This was reported privately by a user and also by another user on the Apache Cassandra users mailing list: https://firstname.lastname@example.org/msg60772.html