using System;

using System.Linq;

using System.Collections.Generic;

​

public class MySuperList

{

    public DateTime MyDate {get; set;}

    public string MyString {get; set;}

}

​

public class Program

{

    public static void Main()

    {

        var lst = new List<MySuperList>();

        lst.Add(new MySuperList{

            MyDate = DateTime.Now,

            MyString = "A"

        });

        lst.Add(new MySuperList{

            MyDate = DateTime.Now,

            MyString = "A"

        });

        lst.Add(new MySuperList{

            MyDate = DateTime.Now,

            MyString = "A"

        });

        lst.ForEach(x=>{

            x.MyDate = new DateTime(x.MyDate.Year, x.MyDate.Month, x.MyDate.Day, x.MyDate.Hour, x.MyDate.Minute, x.MyDate.Second);

        });

        var grouped = lst

            .GroupBy(x=>new{

                x.MyDate,

                x.MyString

            })

            .Where(x=>x.Count()>1)

            .Select(x=>x.Key)

            .ToList();

        grouped.ForEach(x=>{

            Console.WriteLine($"{x.MyDate} - {x.MyString}");

        });

    }

}