internal sealed class Program
    {
        private const int NUMBERS_COUNT = 30 * 1000 * 1000;
        private static readonly Random randomizer = new Random();

        private static void Main()
        {
            var items = new Item[NUMBERS_COUNT];

            for (var i = 0; i < items.Length; i++)
                items[i] = new Item(randomizer.NextDouble(), string.Empty);

            var dict = new Dictionary<long, List<Item>>();
            foreach (var item in items)
            {
                var key = BitConverter.DoubleToInt64Bits(item.Price);
                List<Item> outItems;
                if (!dict.TryGetValue(key, out outItems))
                {
                    outItems = new List<Item>();
                    dict.Add(key, outItems);
                }
                outItems.Add(item);
            }

            foreach (var value in dict.Values)
                value.TrimExcess();

            //consumme in paralel
            // Parallel.ForEach()
        }

        private sealed class Item
        {
            public Item(double price, string someOtherStuff)
            {
                Price = price;
                SomeOtherStuff = someOtherStuff;
            }

            public double Price { get; }
            public string SomeOtherStuff { get; }
        }
    }

...
   int t = Environment.TickCount;
            Parallel.ForEach(dict.Keys, (key) =>
            {
                bool b = dict.ContainsKey(key);
            });
            t = Environment.TickCount - t;
            Console.WriteLine(t);

your_source_here.ToLookup(item => item).ToDictionary(item => item.Key, item => item.Count());