CounterHashSet.cs

using System;
using System.Collections.Generic;

class CounterHashSet<T>
{
    Dictionary<T, int> container;

    public CounterHashSet()
    {
        container = new Dictionary<T, int>();
    }

    public bool Add(T element)
    {
        if (container.ContainsKey(element))
        {
            container[element]++;
            return true;
        }
        container[element] = 1;
        return true;
    }

    public bool Remove(T element)
    {
        if (!container.ContainsKey(element))
        {
            return false;
        }
        if (container[element] == 1)
        {
            container.Remove(element);
            return true;
        }
        container[element]--;
        return true;
    }

    public HashSet<T> ToHashSet()
    {
        return new HashSet<T>(container.Keys);
    }
}