实体框架重复记录

Question

假设我有这门课程：

public class Element
{
    [Key]
    public int ElementId { get; set; }

    public int? ParentId { get; set; }

    [ForeignKey("ParentId")]
    public virtual Element Parent { get; set; }

    [Required, MaxLength(100)]
    public string Name { get; set; }

    public Element(string name, Element parent)
    {
        Name = name;
        Parent = parent;
    }
}

现在我有了

Element A = new Element("A", null);
Element B = new Element("B", A);
Element C = new Element("C", A);

Context.Elements.Add(A); 
Context.Elements.Add(B); 
Context.Elements.Add(C);

EF立即在DB中写入三份A副本。除非它不存在，我如何告诉EF不要写父元素？