By default, the connection TTL is 1 minute. The server resources corresponding to the durable consumer C1 will be held for 1 minute after it was detected that the client was killed. This means indeed that you can not create a durable consumer for the same ID during 1 whole minute.
You can change the TTL value for the connection factory used to create the durable consumers so that the resources are freed faster than that and the durable consumer C2 can take over quickly.
This is a scenario which would basically affect all of us and thus curious about this.
Jeff's answer is correct. This is also discussed in the user manual (see chapter on connection ttl), and has been discussed on several other threads.