Get Unique values at document level using xpath

I have used preceding-sibling and following-sibling to get the unique skill values from the below XML message.

XmlDocument doc = new XmlDocument();
doc.Load(@"{file-uri}");

MemoryStream xmlStream = new MemoryStream();
            doc.Save(xmlStream);

            xmlStream.Flush();//Adjust this if you want read your data
            xmlStream.Position = 0;

            //XmlReader reader = XmlReader.Create(doc.OuterXml);
            //XPathDocument document = new XPathDocument(reader);
            XPathDocument document = new XPathDocument(xmlStream);
            XPathNavigator xPathNavigator = document.CreateNavigator();

            xPathNavigator.MoveToRoot(); // Move to the root element.
            if (xPathNavigator.HasChildren)
            {
                xPathNavigator.MoveToFirstChild();
            }
XmlNamespaceManager namespaceManager = new XmlNamespaceManager(new NameTable());
          nsmgr.AddNamespace("ns0:", "http://www.RVRCompany.com/en/core/employee");

XPathNodeIterator nodIterator = xPathNavigator.Select("//ns0:Employee/ns0:Project/ns0:Skills/ns0:Skill/ns0:SkillName[not(../../../../following-sibling::ns0:Employee/ns0:Project/ns0:Skills/ns0:Skill/ns0:SkillName = .) and not(preceding-sibling::ns0:SkillName=.)]",namespaceManager);

Console.WriteLine("Unique values **********************************************");
            while (nodIterator.MoveNext())
            {
                Console.WriteLine(nodIterator.Current.Value);
            }
            Console.ReadLine();

Sample XML:

<ns0:UpdateEmployeesRequest xmlns:ns0="http://www.RVRCompany.com/en/core/employee">
<ns0:EmployeeDetails>
<ns0:Employee>
<ns0:EmpID>1</ns0:EmpID>
<ns0:Project>
<ns0:ProjectName>PROJ1</ns0:ProjectName>
<ns0:Skills>
<ns0:Skill>
<ns0:SkillType>MS</ns0:SkillType>
<ns0:SkillName>.Net</ns0:SkillName>
</ns0:Skill>
<ns0:Skill>
<ns0:SkillType>MS</ns0:SkillType>
<ns0:SkillName>BizTalk</ns0:SkillName>
</ns0:Skill>
<ns0:Skill>
<ns0:SkillType>MS</ns0:SkillType>
<ns0:SkillName>.Net</ns0:SkillName>
</ns0:Skill>
</ns0:Skills>
</ns0:Project>
</ns0:Employee>
<ns0:Employee>
<ns0:EmpID>2</ns0:EmpID>
<ns0:Project>
<ns0:ProjectName>PROJ2</ns0:ProjectName>
<ns0:Skills>
<ns0:Skill>
<ns0:SkillType>MS</ns0:SkillType>
<ns0:SkillName>.Net</ns0:SkillName>
</ns0:Skill>
<ns0:Skill>
<ns0:SkillType>MS</ns0:SkillType>
<ns0:SkillName>BizTalk</ns0:SkillName>
</ns0:Skill>
<ns0:Skill>
<ns0:SkillType>MS</ns0:SkillType>
<ns0:SkillName>.Net</ns0:SkillName>
</ns0:Skill>
<ns0:Skill>
<ns0:SkillType>Sun Micros</ns0:SkillType>
<ns0:SkillName>Java</ns0:SkillName>
</ns0:Skill>
</ns0:Skills>
</ns0:Project>
</ns0:Employee>
<ns0:Employee>
<ns0:EmpID>3</ns0:EmpID>
<ns0:Project>
<ns0:ProjectName>PROJ2</ns0:ProjectName>
<ns0:Skills>
<ns0:Skill>
<ns0:SkillType>MS</ns0:SkillType>
<ns0:SkillName>VB6</ns0:SkillName>
</ns0:Skill>
<ns0:Skill>
<ns0:SkillType>MS</ns0:SkillType>
<ns0:SkillName>.Net</ns0:SkillName>
</ns0:Skill>
<ns0:Skill>
<ns0:SkillType>MS</ns0:SkillType>
<ns0:SkillName>SQL Server</ns0:SkillName>
</ns0:Skill>
<ns0:Skill>
<ns0:SkillType>MS</ns0:SkillType>
<ns0:SkillName>IIS</ns0:SkillName>
</ns0:Skill>
<ns0:Skill>
<ns0:SkillType>Sun Micros</ns0:SkillType>
<ns0:SkillName>Linux</ns0:SkillName>
</ns0:Skill>
</ns0:Skills>
</ns0:Project>
</ns0:Employee>
<ns0:Employee>
<ns0:EmpID>3</ns0:EmpID>
<ns0:Project>
<ns0:ProjectName>PROJ2</ns0:ProjectName>
<ns0:Skills>
<ns0:Skill>
<ns0:SkillType>MS</ns0:SkillType>
<ns0:SkillName>VB6</ns0:SkillName>
</ns0:Skill>
<ns0:Skill>
<ns0:SkillType>MS</ns0:SkillType>
<ns0:SkillName>IIS</ns0:SkillName>
</ns0:Skill>
<ns0:Skill>
<ns0:SkillType>Sun Micros</ns0:SkillType>
<ns0:SkillName>Linux</ns0:SkillName>
</ns0:Skill>
<ns0:Skill>
<ns0:SkillType>Sun Micros</ns0:SkillType>
<ns0:SkillName>Tomcat</ns0:SkillName>
</ns0:Skill>
<ns0:Skill>
<ns0:SkillType>Sun Micros</ns0:SkillType>
<ns0:SkillName>BizTalk</ns0:SkillName>
</ns0:Skill>
</ns0:Skills>
</ns0:Project>
</ns0:Employee>
</ns0:EmployeeDetails>

</ns0:UpdateEmployeesRequest>