Try this code, it would work for you to get the only elements that are duplicated in the List.
List<int> list = new List<int>() { 1, 3, 3, 4, 5, 6, 6, 7, 8 };
foreach (int i in list.GroupBy(s => s)
.SelectMany(grp => grp.Skip(1)))
{
Console.WriteLine(i);
}
.. this will provide you with the only elements that are distinct. In the above context 3 and 6 were printed on screen.