using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<string> list = new List<string>(){"apple", "banana", "apple", "orrange", "banana", "biwa"};

        string dups = list

            .Select((e,i)=>new {e,i})

            .GroupBy(ei=>ei.e)

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

            .SelectMany(gr=>gr.Select(ei=> (ei.i + 1) + "「" + ei.e + "」"))

            .Aggregate((a,b)=>a + ", " + b);

        System.Console.WriteLine(dups + "が重複しています。");

    }

}