Lotto unieke getallen, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            int getal;
            int rounds = 1;
            Random rnd = new Random();
            Console.WriteLine("De 6 LOTTO-getallen zijn:");
            string seenNumbers = ":";
            
            getal = rnd.Next(1,46);
            seenNumbers = string.Concat(seenNumbers,getal,":");
            
            do{
                if(rounds != 1){
                    getal = rnd.Next(1,46);
                    while(seenNumbers.Contains(string.Format(":{0}:", getal))){
                        getal = rnd.Next(1,46);
                    };
                    seenNumbers = string.Concat(seenNumbers,getal,":");      
                }
                Console.WriteLine("Getal: {0}", getal);
                rounds++;
            }
            while(rounds <= 6);
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,14 sec, absolute running time: 0,09 sec, cpu time: 0,08 sec, average memory usage: 11 Mb, average nr of threads: 2