Comment 3 for bug 1863190

What I see in my cloud is that one of the two will schedule and build, and the other will schedule, but fail to build with a rescheduling error:

```
{'message': 'Build of instance 417e19c2-e2a5-48e0-8ce5-0f087c5f6091 was re-scheduled: Anti-affinity instance group policy was violated.', 'code': 500, 'details': 'Traceback (most recent call last):\n File "/opt/openstack/venv/nova/lib/python2.7/site-packages/nova/compute/manager.py", line 1941, in _do_build_and_run_instance\n filter_properties, request_spec)\n File "/opt/openstack/venv/nova/lib/python2.7/site-packages/nova/compute/manager.py", line 2230, in _build_and_run_instance\n instance_uuid=instance.uuid, reason=six.text_type(e))\nRescheduledException: Build of instance 417e19c2-e2a5-48e0-8ce5-0f087c5f6091 was re-scheduled: Build of instance 417e19c2-e2a5-48e0-8ce5-0f087c5f6091 was re-scheduled: Anti-affinity instance group policy was violated.\n', 'created': '2020-02-21T03:43:18Z'}
```

This is with hard anti-affinity.
With soft-anti-affinity, there would be no reschedule forced, and therefore it would just never be effective.