Show multiple items selected within combobox in Silverlight when do key navigation in edit mode without dropdown open

Question

Just bind ItemSource to Silverlight ComboBox. Do key navigation within a combobox where DropDown should not be opened. after completing key navigation click drop down icon to view the drop down list. There are multiple items selected that has same value, some times different value selected.

Is there any way to overcome this issue ? Or is that framework issue?

Details:

My combobox xaml is here:

<ComboBox ItemsSource="{Binding Path=ComboBoxItemsSource}" Grid.Column="1" Width="150" Height="40"/> 

where ComboBoxItemsSource is List of String collection that defined in ViewModel.

ViewModel

    string[] productName = new string[]
    {
        "Alice Mutton", 
        "NuNuCa Nuß-Nougat-Creme",  
        "Boston Crab Meat", 
        "Raclette Courdavault", 
        "Wimmers gute Semmelknödel",
        "Gorgonzola Telino",    
        "Chartreuse verte", 
        "Fløtemysost",  
        "Carnarvon Tigers", 
        "Thüringer Rostbratwurst",  
        "Vegie-spread", 
        "Tarte au sucre",   
        "Konbu",    
        "Valkoinen suklaa", 
        "Queso Manchego La Pastora",    
        "Perth Pasties",    
        "Vegie-spread", 
        "Tofu", 
        "Sir Rodney's Scone 7", 
        "Manjimup Dried Apples"
    };

    private List<string> _comboBoxItemsSource = new List<string>();
    public List<string> ComboBoxItemsSource
    {
        get { return _comboBoxItemsSource; }
        set { _comboBoxItemsSource = value; }
    }

    public ViewModel()
    {
       _comboBoxItemsSource = productName.ToList();
    } 

The Setting should be Button, ComboBox.

• First get focus on Button.
• Then press tab to focus combo box.
• Now, Just do pressing - right/left/up/down keys continuously.
• Now click on drop down icon. you can see that multiple items
  selected.