想參考我的解法，請先去VS裡的nuget安裝epplus免費的Excel讀寫工具(效能比office快很多，因為是純xml讀寫，上千行的xlsx檔讀寫小於10秒，執行時期可免安裝office)

using OfficeOpenXml;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Windows.Forms;

        private void Form1_Load(object sender, EventArgs e)
        {
            ExcelPackage.LicenseContext = LicenseContext.NonCommercial;
            using (ExcelPackage ep = new ExcelPackage(new System.IO.FileInfo(@"s:\test\test.xlsx")))
            {
                // 準備收集所有資料用
                var dataOrigin = new List<KeyValuePair<string, int>>();
                // 範例當然是放在第一個工作表
                ExcelWorksheet sh = ep.Workbook.Worksheets[0];
                // 先讀取這個xlsx檔有多少列
                int totalRows = sh.Dimension.Rows; 
                for (int r = 2; r <= totalRows;r++)
                {
                    dataOrigin.Add(new KeyValuePair<string, int>(sh.Cells[r, 1].Value.ToString(),Convert.ToInt32(sh.Cells[r, 2].Value)));
                }
                // 清空已經存在的轉換後結果
                sh.Cells[2, 4, totalRows, 5].Clear(); 

                // 把收集到的資料以編號做成群組(GroupBy)並按編號排序(OrderBy)
                var dataOutput = dataOrigin.GroupBy(g => g.Key).OrderBy(s => s.Key);

                int R = 2; //因為用foreach所以自己加index
                foreach(var d in dataOutput)
                {
                    sh.Cells[R, 4].Value = d.Key;  // 輸出編號
                    sh.Cells[R, 5].Value = d.Sum(s => s.Value);  // 輸出這個編號的加總
                    R++;
                }
                ep.Save();
            }
        }

輸出前

輸出結果

其實你要的是加總吧,
不懂你的問題在哪,
既然你都用List來存資料了,
就做完計算之後,
再一列一列把資料寫出來就好了.

更新回答

我照著你的輸入，發現好像格子為NULL，會無法處理

我照你更新的 excel 做處理，沒問題

void Main()
{
	var path = @"C:\Users\Wei\Downloads\TestIssue232.xlsx";
	var rows = MiniExcel.Query(path,useHeaderRow:true);
	var newInput = (from s in rows
						group s by s.Number into g
						select new {
							Number=g.Key,
							Apr=g.Sum(_=>(decimal?)_.Apr),
							May=g.Sum(_=>(decimal?)_.May),
							Jun=g.Sum(_=>(decimal?)_.Jun),
						}
	);
	Console.WriteLine(newInput);
	
	MiniExcel.SaveAs("output.xlsx",newInput);
}

原回答

這樣嗎?

可以嘗試使用我寫的一個小工具 MiniExcel

安裝 nuget 包

Install-Package MiniExcel -Version 0.13.3

代碼

void Main()
{
	var path = @"C:\Users\Wei\Downloads\Book5.xlsx";
	var rows = MiniExcel.Query(path,true);

	var outputPath = "output2.xlsx";
	MiniExcel.SaveAs(outputPath, rows.GroupBy(g => g.號碼).Select(s => new {號碼=s.Key,金額=s.Sum(_=>(int)_.金額)}));
}

但牽扯到修改樣式建議還是使用 epplus

參考這篇拿去改
原始程式碼在此

using System;
using System.Collections.Generic;
using System.Linq;

public class Program
{
    public static void Main()
    {
        var transactions = new List<Transaction>{};

		transactions.Add(new Transaction { Category = "1", Amount = 100 });
		transactions.Add(new Transaction { Category = "2", Amount = 200 });
		transactions.Add(new Transaction { Category = "2", Amount = 100 });
		transactions.Add(new Transaction { Category = "3", Amount = 300 });
		transactions.Add(new Transaction { Category = "4", Amount = 100 });
		transactions.Add(new Transaction { Category = "5", Amount = 400 });
		transactions.Add(new Transaction { Category = "6", Amount = 100 });
		transactions.Add(new Transaction { Category = "5", Amount = 400 });
		transactions.Add(new Transaction { Category = "1", Amount = 100 });
		transactions.Add(new Transaction { Category = "2", Amount = 200 });
		
        var summaryApproach1 = transactions.GroupBy(t => t.Category)
                           .Select(t => new
                           {
                               Category = t.Key,
                               Amount = t.Sum(ta => ta.Amount),
                           }).ToList();

		Console.WriteLine("-- Summary: Approach 1 --");
        summaryApproach1.ForEach(
            row => Console.WriteLine($"Category: {row.Category}, Amount: {row.Amount}"));



        var summaryApproach2 = transactions.GroupBy(t => t.Category, (key, t) =>
        {
            var transactionArray = t as Transaction[] ?? t.ToArray();
            return new
            {
                Category = key,
                Amount = transactionArray.Sum(ta => ta.Amount),
            };
        }).ToList();

        Console.WriteLine("-- Summary: Approach 2 --");
        summaryApproach2.ForEach(
            row => Console.WriteLine($"Category: {row.Category}, Amount: {row.Amount}"));
    }
}

public class Transaction
{
    public string Category { get; set; }
    public decimal Amount { get; set; }
}