Sum of duplicated items Tuple C#

Question

I have a data in a tuple C# I will calcaulate the sum of the last column grouped by month. my program gives wrong results as shown in the table below. 

 

The program  is here :

var q1 = from x in Vinst
                    group x by x.Item1 into g
                    let count = g.Count()
                    orderby count descending
                    select new { Value = g.Key, Count = count};

            double sum = 0.0;//= (from x in Vinst where x.Item1.Equals("augusti") select x.Rest.Item2).Sum();
            int k = 0;
            foreach (var lst in Vinst)
            {
                sum += lst.Rest.Item2; k++;
                richTextBox7.AppendText("Vinst Data" + "\t" + lst.Item1 + "\t" + lst.Item2 + "\t" + lst.Item3 + "\t" + lst.Item4 + "\t" + lst.Item5 + "\t" + lst.Item6 +"\t" + lst.Item7+ "\t" + lst.Rest.Item1 + "\t" + lst.Rest.Item2 + "\n");

                foreach (var x in q1)
                    {
                        if (k == x.Count)
                            richTextBox7.AppendText("\nData summa" + " " + sum + "\n");
                    }
            }

july     GBP     6000    13        400    19500    1500    5200    800

                                                                           Data sum 800

july     SAR     1000    2          400    1200    600    800    200

july     USD     4500    8.95    500    15993    1787 4475    25

                                                                        Data sum 1025

august USD     5700    8.5    600    5950       700    5100    600

 

The correct result should be like this

 

july     GBP     6000    13     400    19500    1500    5200    800

july     SAR     1000    2       400    1200       600    800       200

july     USD     4500    8.95  500    15993    1787    4475    25

                                                                           Data sum 1025

august USD    5700    8.5    600    5950       700    5100    600

                                                                              data sum 600