using System;

using System.Linq;

using System.Text.RegularExpressions;

using System.Globalization;

using System.Collections.Generic;

public class Program

{

    private static readonly Regex regex = new Regex(@"^(?<num>\d+)\.", RegexOptions.Compiled);

    public static void Main()

    {

        var list = new [] {

            "1.one",

            "2. two",

            "no number",

            "2.duplicate",

            "300. three hundred",

            "4-ignore this"

        };

        var distinctWithNumbers = list.Where(s => regex.IsMatch(s)).GroupBy(s => regex.Match(s).Groups["num"].Value).Select(g => g.First());

        distinctWithNumbers.ToList().ForEach(Console.WriteLine);

        //Console.ReadKey();

    }

}