哈希集中的重复元素

我现在对哈希集有个问题。我有一些不可变的类,它们只包含一个项,当我向一个hashset中添加两个不同的类,它们的数据相同时,我就在这个集中得到了它们。这很奇怪,因为我在基类和超类上都重载了Equals和GetHashCode。

public abstract class Contact :IEquatable<Contact>
{
    public readonly BigInteger Id;

    public Contact(BigInteger id) { this.Id = id; }

    public abstract bool Equals(Contact other);

    public abstract int GetHashCode();

    public abstract bool Equals(object obj);
}

以及继承类:

public class KeyOnlyContact :Contact, IEquatable<KeyOnlyContact>
{
    public KeyOnlyContact(BigInteger id) :base(id) { }

    public override bool Equals(object obj)
    {
        if (obj is KeyOnlyContact)
            return Equals(obj as KeyOnlyContact);
        else if (obj is Contact)
            return Equals(obj as Contact);
        else
            return (this as object).Equals(obj);
    }

    public override bool Equals(Contact other)
    {
        if (other is KeyOnlyContact)
            return Equals(other as KeyOnlyContact);
        else
            return (this as object).Equals(other as object);
    }

    public bool Equals(KeyOnlyContact other)
    {
        return other.Id.Equals(Id);
    }

    public override int GetHashCode()
    {
        return Id.GetHashCode();
    }

澄清:

BigInteger a;
HashSet<Contact> set;

set.add(new KeyOnlyContact(a));
set.add(new KeyOnlyContact(a));

set.Count == 2