Display selections in CheckBox ComboBox

I have a comboBox which get populated by an XML. It lists every item as checkbox item I would like the ComboBox to display the checked selections, like the image below

This is my XML

<?xml version="1.0" encoding="utf-8" ?>
<ComboBox>
  <Customer name="John">
    <Data>
      <System>Linux</System>
    </Data>
  </Customer>
  <Customer name="Fernando">
    <Data>
      <System>Microsoft</System>
      <System>Mac</System>
    </Data>
  </Customer>
</ComboBox>

This is my XAML

<ComboBox x:Name="customer_comboBox" HorizontalAlignment="Left" Margin="83,259,0,0" VerticalAlignment="Top" Width="172" SelectionChanged="customer_comboBox_SelectionChanged" >
     <ComboBox.ItemTemplate>
         <DataTemplate>
                <CheckBox Content="{Binding}"/>
        </DataTemplate>
     </ComboBox.ItemTemplate>
</ComboBox>

And this is the code that is used to populate the customer_comboBox

 XmlDocument doc = new XmlDocument();
 doc.Load(Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments) +@"\comboBox.xml");
 XmlNodeList customerList = doc.SelectNodes("ComboBox/Customer");
 List<string> customers = new List<string>();

 foreach (XmlNode node in customerList)
 {
     customers.Add(node.InnerText);
 }
 customer_comboBox.ItemsSource = customers;