Поиск одинаковых элементов массива

List<Good> arr = new List<Good>();
            Good a = new Good { price = 10, date = "10.10.2017" };
            Good b = new Good { price = 1, date = "10.10.2017" };
            Good c = new Good { price = 2, date = "10.10.2017" };
            Good d = new Good { price = 3, date = "11.10.2017" };
            Good e = new Good { price = 5, date = "11.10.2017" };
            Good p = new Good { price = 5, date = "12.10.2017" };
            Good o = new Good { price = 5, date = "13.10.2017" };
            arr.Add(a);
            arr.Add(b);
            arr.Add(c);
            arr.Add(d);
            arr.Add(e);
            arr.Add(p);
            arr.Add(o);

Dictionary<string, int> totalPriceByDate = arr.GroupBy(x => x.date)
                                              .ToDictionary(x => x.Key, x=> x.Sum());

var resultAsArray = totalPriceByDate.Select(x => string.Format(@"{1}:""{0}""", x.Key, x.Value)).ToArray();

Dictionary<string, int> totalPriceByDate = arr.GroupBy(x => x.date, x => x.price)
                                              .ToDictionary(x => x.Key, x=> x.Sum());

List<Good> arr = new List<Good>();
            arr.Add(new Good { price = 45, date = "2009-01-01" });
            arr.Add(new Good { price = 25, date = "2009-01-01" });
            arr.Add(new Good { price = 50, date = "2009-01-02" });
            arr.Add(new Good { price = 32, date = "2009-01-02" });
            arr.Add(new Good { price = 45, date = "2009-01-03" });
            arr.Add(new Good { price = 25, date = "2009-01-03" });
            arr.Add(new Good { price = 50, date = "2009-02-01" });
            arr.Add(new Good { price = 32, date = "2009-02-01" });
            arr.Add(new Good { price = 45, date = "2009-02-01" });
            arr.Add(new Good { price = 25, date = "2009-02-01" });
            arr.Add(new Good { price = 50, date = "2009-03-02" });
            arr.Add(new Good { price = 32, date = "2009-03-02" });
            arr.Add(new Good { price = 45, date = "2009-04-03" });
            arr.Add(new Good { price = 25, date = "2009-04-03" });
            arr.Add(new Good { price = 50, date = "2009-05-01" });
            arr.Add(new Good { price = 32, date = "2009-05-01" });
            arr.Add(new Good { price = 45, date = "2009-06-01" });