public void Adding_duplicate_item_to_the_binding_list_is_ignored()
{
var item = new ListElement(4);
var oc = new ObservableHashSet <ListElement> {
3, 1, item, 1, 5, 9
};
var obbl = new ObservableBackedBindingList <ListElement>(oc);
obbl.Add(item);
Assert.Equal(6, oc.Count);
Assert.Equal(1, oc.Count(i => ReferenceEquals(i, item)));
}
