using System.Text.RegularExpressions;

using System;

public class Program

{

    public static void Main(){

        //update to correct regex

        Regex regex = new Regex(@"\b(?<word>\w+)\s+(\k<word>)\b", RegexOptions.Compiled | RegexOptions.IgnoreCase);     

        string text = "The quick brown fox fox jumps over the lazy dog dog";

        MatchCollection matches = regex.Matches(text);

        //private MatchCollection matches;// = regex.Matches(text);

        Console.WriteLine("Match count " + matches.Count);

        //GroupCollection gc = matches[0].Groups;

        //matches.ForEach(match => Console.WriteLine("Match word " + match.Groups["word"].Value));

        foreach(Match match in matches){

            GroupCollection groups = match.Groups;

            Console.WriteLine("Match word " + groups["word"].Value);

            //Console.WriteLine(groups[0].Index);

            foreach(Group group_ in groups)

                Console.WriteLine("Position " + group_.Index);

            //match.Foreach()

        }

    }

}