using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var dict = new Dictionary<string, Player>() {

            {"abc", new Player() {GameID = 1, Index = "abc"}},

            {"bca", new Player() {GameID = 1, Index = "bca"}},

            {"fff", new Player() {GameID = 2, Index = "fff"}},

            {"abcc", new Player() {GameID = 3, Index = "abcc"}},

            {"bcca", new Player() {GameID = 3, Index = "bcca"}},

        };

        var result = dict.Values.GroupBy(x => x.GameID).Where(x => x.Count() > 1).Select(g => g.First());

        foreach(var item in result)

            Console.WriteLine(item.GameID);

    }

}

​

public class Player {

    public int GameID {get; set;}

    public string Index {get; set;}

}

​

​