An instance action should be unique per instance uuid and request ID. That's pointed out in the data model:
https://github.com/openstack/nova/blob/master/nova/db/sqlalchemy/models.py#L1241
"The intention is that there will only be one of these per user request. A lookup by (instance_uuid, request_id) should always return a single result."
It's also enforced in the API:
https://github.com/openstack/nova/blob/master/nova/db/sqlalchemy/api.py#L5719
def action_get_by_request_id(context, instance_uuid, request_id):
"""Get the action by request_id and given instance."""
action = _action_get_by_request_id(context, instance_uuid, request_id)
return action
But that is not actually enforced in the schema using a UniqueConstraint.
This is a low priority but it's technically something we should have in the data model/schema.
Note that any database migration which adds a UniqueConstraint here would have to check for duplicates and drop them before attempting to create the constraint.