For the custom model, a copy_relations(self, oldinstance)
method must be defined as part of the model. In this case, you'd use something like:
def copy_relations(self, oldinstance):
self.authorized_personnel = oldinstance.authorized_personnel.all()
This provides the information that Django needs when it makes a published copy of the plugin instance. For more information, see the documentation: http://docs.django-cms.org/en/latest/extending_cms/custom_plugins.html#handling-relations