using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var casingMismatch = new Dictionary<string, string>();

        var list1 = new List<string> {"Aaa", "bb", "aaa", "dd"};

        foreach (var s in list1) {

            var match = list1.FirstOrDefault(x => x.Equals(s, StringComparison.CurrentCultureIgnoreCase));

            if (match != null && !s.Equals(match))

            {

                casingMismatch.Add(s, match);

            };

        }

        foreach (var mismatch in casingMismatch)

        {

            Console.WriteLine(mismatch);

        }

    }

}