List转换字典去重问题

var list = new List<TestClass>
{
    new TestClass{Id=1,Name="1"},
    new TestClass{Id=2,Name="2"},
    new TestClass{Id=3,Name="3"},
    new TestClass{Id=4,Name="4"},
    new TestClass{Id=5,Name="5"},
    new TestClass{Id=1,Name="9"},
};

var hash = new HashSet<int>();
var dict = list.Where(n=>!hash.Add(n.Id)).ToDictionary(key => key.Id, value => value.Name);