using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var now = DateTime.Now;

        var tomorrow = DateTime.Now.AddDays(1);

        var list = new List<(string SkuCode, DateTime Date)>(){

            ("SKU1", now)

            , ("SKU2", now)

            , ("SKU2", now)

            , ("SKU3", tomorrow)

            , ("SKU2", tomorrow)

        };

        var duplicateQuantities = list.GroupBy(d => new { d.SkuCode, d.Date }).Where(g => g.Count() > 1).ToList();

        if (duplicateQuantities.Any())

        {

            var details = string.Join(',', duplicateQuantities.Select(g => $"SKU {g.Key} - {g.Count()} values"));

            Console.WriteLine(details);

        }

    }

}