You're correct that Ceilometer and Designate will "race" to consume the messages, the correct configuration is to provide Nova with two different notification topics, causing it to emit each notification twice, and allocate one of the topics to Ceilometer, the other to Designate.
You're correct that Ceilometer and Designate will "race" to consume the messages, the correct configuration is to provide Nova with two different notification topics, causing it to emit each notification twice, and allocate one of the topics to Ceilometer, the other to Designate.
We should update our docs to clarify this.