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CSharp · Console

/Program.cs

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;}
}