C# - Identify Duplicate Nodes In XPath 1.0 Using A XPathNavigator To Evaluate?

I am trying to identify duplicate serial numbers from the following xml using XPath 1.0 and then evaluating it in .Net using an XPathNavigator.

<?xml version="1.0" encoding="utf-16"?>
<Inventory xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<Items>

[code]....

I tried to do this by evaluating this

//Items/Item/SerialNumber

expression in a custom XSLT Context Function (implementing IXsltContextFunction like this MSDN example) in .Net but the Invoke function gets called one result at a time so I have no visibility of the other results to find duplicates.

1) Is there a way of doing this using a single XPath 1.0 expression?

2) Is there a way of passing in an array of elements into a single Invoke call of the custom XSLT Context Function class? I'm working in VB.Net but am happy with any C# examples anyone can share.