If you have mapped a view or a synonym, the SchemaUpdate tries to create them as a table and fails because an object with the same name already exists. That should not happend.
you need to use schema-action="none" attribute on your <class> mapping/IClassMapper. if you want to map to a view rather than a table. Please see here: http://nhibernate.info/blog/2008/12/16/customizing-schema-export-with-schema-action.html