Multiset

A multiset is like a normal set, but it allows you to have more than one of the same item. For example, you could have a multiset with the numbers 1, 2, and 3, but you could also have another multiset with three 2s, which would look like 2, 2, 2. In a regular set, the numbers 2, 2, 2 would all be counted as 1.