Nagios gets duplicate hostnames when re-adding cross model relations

[Connor Chamberlain]

I saw Nagios pick up a duplicate of every hostname in my openstack deployment after I redeployed openstack without removing nagios.

My original deployment involved adding nagios after all of openstack was deployed, and nagios used a cross model relation to attach to all the nrpe charms in the openstack model. This openstack model later had to be torn down and redeployed. When I rebuilt the cross model relation, Nagios had a duplicate of every hostname.