using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var initialDict = new Dictionary<int, string>{

            {1, "Value"},

            {2, "Value"},

            {3, "Value"},

            {4, "Value"},

            {5, "Value2"}

        };

        var newDict = initialDict.Select(x=>x.Value).Distinct().ToDictionary(x=>x, x=> initialDict.Where(kvp=>kvp.Value == x).Select(kvp=>kvp.Key));

        Console.WriteLine(string.Join("\n", newDict.Select(x=>x.Key +": {"+ string.Join(", ", x.Value)+"}")));

    }

}