I just hit this in a charm gate job:
2020-07-23 10:37:15 ERROR juju-log cloud-compute:22: Cell1 discover_hosts failed
b"Getting computes from cell 'cell1': ef421f8f-5182-4014-bb1f-b9a93ebec5a5\nChecking host mapping for compute host 'juju-e09d65-zaza-c69af9d26505-8.project.serverstack': 8b52ad0c-4cf8-4c7d-984b-3283de75be4f\nCreating host mapping for compute host 'juju-e09d65-zaza-c69af9d26505-8.project.serverstack': 8b52ad0c-4cf8-4c7d-984b-3283de75be4f\nChecking host mapping for compute host 'juju-e09d65-zaza-c69af9d26505-6.project.serverstack': 54d39de2-671b-4c46-b31e-4e80202e1c27\nCreating host mapping for compute host 'juju-e09d65-zaza-c69af9d26505-6.project.serverstack': 54d39de2-671b-4c46-b31e-4e80202e1c27\nERROR: Duplicate host mapping was encountered. This command should be run once after all compute hosts have been deployed and should not be run in parallel. When run in parallel, the commands will collide with each other trying to map the same hosts in the database at the same time. Error: Host 'juju-e09d65-zaza-c69af9d26505-6.project.serverstack' mapping already exists\n"
2020-07-23 10:37:15 DEBUG cloud-compute-relation-changed Traceback (most recent call last):
2020-07-23 10:37:15 DEBUG cloud-compute-relation-changed   File "/var/lib/juju/agents/unit-nova-cloud-controller-0/charm/hooks/cloud-compute-relation-changed", line 1383, in <module>
2020-07-23 10:37:15 DEBUG cloud-compute-relation-changed     main()
2020-07-23 10:37:15 DEBUG cloud-compute-relation-changed   File "/var/lib/juju/agents/unit-nova-cloud-controller-0/charm/hooks/cloud-compute-relation-changed", line 1375, in main
2020-07-23 10:37:15 DEBUG cloud-compute-relation-changed     hooks.execute(sys.argv)
2020-07-23 10:37:15 DEBUG cloud-compute-relation-changed   File "/var/lib/juju/agents/unit-nova-cloud-controller-0/charm/charmhelpers/core/hookenv.py", line 945, in execute
2020-07-23 10:37:15 DEBUG cloud-compute-relation-changed     self._hooks[hook_name]()
2020-07-23 10:37:15 DEBUG cloud-compute-relation-changed   File "/var/lib/juju/agents/unit-nova-cloud-controller-0/charm/hooks/cloud-compute-relation-changed", line 674, in cloud_compute_relation_changed
2020-07-23 10:37:15 DEBUG cloud-compute-relation-changed     add_hosts_to_cell_when_ready()
2020-07-23 10:37:15 DEBUG cloud-compute-relation-changed   File "/var/lib/juju/agents/unit-nova-cloud-controller-0/charm/hooks/cloud-compute-relation-changed", line 689, in add_hosts_to_cell_when_ready
2020-07-23 10:37:15 DEBUG cloud-compute-relation-changed     ncc_utils.add_hosts_to_cell()
2020-07-23 10:37:15 DEBUG cloud-compute-relation-changed   File "/var/lib/juju/agents/unit-nova-cloud-controller-0/charm/hooks/nova_cc_utils.py", line 954, in add_hosts_to_cell
2020-07-23 10:37:15 DEBUG cloud-compute-relation-changed     subprocess.check_output(cmd)
2020-07-23 10:37:15 DEBUG cloud-compute-relation-changed   File "/usr/lib/python3.6/subprocess.py", line 356, in check_output
2020-07-23 10:37:15 DEBUG cloud-compute-relation-changed     **kwargs).stdout
2020-07-23 10:37:15 DEBUG cloud-compute-relation-changed   File "/usr/lib/python3.6/subprocess.py", line 438, in run
2020-07-23 10:37:15 DEBUG cloud-compute-relation-changed     output=stdout, stderr=stderr)
2020-07-23 10:37:15 DEBUG cloud-compute-relation-changed subprocess.CalledProcessError: Command '['nova-manage', 'cell_v2', 'discover_hosts', '--cell_uuid', 'ef421f8f-5182-4014-bb1f-b9a93ebec5a5', '--verbose']' returned non-zero exit status 2.
2020-07-23 10:37:15 ERROR juju.worker.uniter.operation runhook.go:132 hook "cloud-compute-relation-changed" failed: exit status 1

Complete set of artifacts: https://openstack-ci-reports.ubuntu.com/artifacts/test_charm_pipeline_func_smoke/openstack/charm-octavia/742459/2/18272/index.html