Removing Duplicate Items from Xml using XPath

I needed this today, and I figured I will need it again in the future, hence my need to blog this.

I need some XPath which would remove duplicate elements from xml depending on an attribute on the item, e.g.

<Items>
   <Item ItemNumber=”1”/>
   <Item ItemNumber=”1”/>
   <Item ItemNumber=”2”/>
</Items>

The following XPath does the job perfectly (for my needs)

//Items/Item[not(@ItemNumber=preceding::Item/@ItemNumber)]/@ItemNumber

Which returns the following list:

<Items>
   <Item ItemNumber=”1”/>
   <Item ItemNumber=”2”/>
</Items>