运用linq查找所有重复的元素

如题：

有一个List<string>类型的List<T>

List<String> list = new List<String>{"6","1","2","4","6","5","1"};`

需要返回结果List包含 {"6","1"}

代码如下：

var duplicates = lst.GroupBy(s => s)
    .SelectMany(grp => grp.Skip(1));