Find all intersecting data, not just the unique values

Question

I thought that I understood Intersect, but it turns out I was wrong.

 List list1 = new List() { 1, 2, 3, 2, 3};
 List<         


        

Answer 1

var set = new HashSet(list1.Intersect(list2));
return list1.Concat(list2).Where(i=>set.Contains(i));