public void CanClearDuplicateFilter()
{
ContentItem item = CreateOneItem <FirstItem>(1, "one", null);
ItemList list = new ItemList();
list.Add(item);
list.Add(item);
list.Add(item);
DuplicateFilter filter = new DuplicateFilter();
filter.Filter(list);
Assert.AreEqual(1, list.Count);
filter.Clear();
filter.Filter(list);
Assert.AreEqual(1, list.Count);
}