How do I do an integer list intersection while keeping duplicates?

ILookup<int, int> lookup1 = list1.ToLookup(i => i);
ILookup<int, int> lookup2 = list2.ToLookup(i => i);

int[] result =
(
  from group1 in lookup1
  let group2 = lookup2[group1.Key]
  where group2.Any()
  let smallerGroup = group1.Count() < group2.Count() ? group1 : group2
  from i in smallerGroup
  select i
).ToArray();

The where expression is technically optional, I feel it makes the intent clearer.

---

If you want more terse code:

ILookup<int, int> lookup2 = list2.ToLookup(i => i);

int[] result =
(
  from group1 in list1.GroupBy(i => i)
  let group2 = lookup2[group1.Key]
  from i in (group1.Count() < group2.Count() ? group1 : group2)
  select i
).ToArray();

I wrote this extension to solve the problem:

public static IEnumerable<T> Supersect<T>(this IEnumerable<T> a, ICollection<T> b)
              => a.Where(b.Remove);

example:

var a = new List<int> { 1, 2, 2, 2, 3, 3, 4, 5 };
var b = new List<int> { 1, 1, 2, 2, 3, 3, 3, 4, 4};

var result = a.Supersect(b);

result:

{ 1, 2, 2, 3, 3, 4 }