using System;

using System.Collections.Generic;

using System.Linq;          

public class Program

{

    public static void Main()

    {

        var tools = new List<Tool>()

            {

                new Tool {Id=444, Number=121, Source="Master"},

                new Tool {Id=777, Number=121, Source="M1"},

                new Tool {Id=333, Number=51, Source="Master"},

                new Tool {Id=555, Number=61, Source="Master"},

                new Tool {Id=848, Number=51, Source="M1"}

            };

         List<Tool> distinctTool = tools.GroupBy(p => p.Number).Select(f => f.OrderByDescending(q => q.Source == "M1").First()).ToList();

        foreach(var Too in distinctTool)

        { 

          Console.WriteLine("Number" +Too.Number);

          Console.WriteLine("Source "+Too.Source);

        }

    }

    public class Tool

    {

        public int Id { get; set; }

        public int Number { get; set; }

        public string Source { get; set; }

   }

}