How to get distinct/unique values from an Xpath query in XAML?

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XAML

WPF

Hi,
Is it possible to get unique values from an xpath query of an xml file in XAML/WPF?
This is the XML file (CurvesIDF.xml):

<?xml version="1.0" encoding="utf-8"?>
<CurvesIDF xmlns="">
  <Curve Name="1">
    <Country>Portugal</Country>
    <Zone>A</Zone>
    <a>5</a>
    10
    <T>5</T>
  </Curve>
  <Curve Name="2">
    <Country>Portugal</Country>
    <Zone>A</Zone>
    <a>45</a>
    -15
    <T>10</T>
  </Curve>
</CurvesIDF>

And this is my XAML markup:

<Window x:Class="CurvesIDF"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <Window.Resources>
        <XmlDataProvider x:Key="CurvesIDF" Source="CurvesIDF.xml" XPath="CurvesIDF/Curve"/>
    </Window.Resources>

    <Grid DataContext="{Binding Source={StaticResource CurvesIDF}}">
        <Grid.ColumnDefinitions>
            <ColumnDefinition />
            <ColumnDefinition />
        </Grid.ColumnDefinitions>
        <ListBox x:Name="listBox" ItemsSource="{Binding XPath=/CurvesIDF/Curve}" DisplayMemberPath="Zone"/>
    </Grid>
</Window>

The output of this is a list with two entries: "A" and "A", because there are two curves with the same Zone value. I would like to be able to get a list with only one "A" value.

Thanks