Adding workaround for libxml2-2.6.26 xpath dup issue

Nokogiri wraps libxml2, which on some old versions (noticed on 2.6.26 on CentOS 5) has a bug causing xpath queries to return duplicate elements under some circumstances.

see https://github.com/sparklemotion/nokogiri/issues/246

This caused api_spec to fail, so fixed by removing duplicate elements from array before extracting the values