Trying to create a random number generate. If number exits in database generate again and add value to database

I am trying to create a random number generator, if the random number exists in the database loop through the block of code again until it finds a number that does not exist. Save value to database. Any help on this would be great thank you.

public IHttpActionResult BookMatch(int id)
        {
            Guid guid = Guid.NewGuid();

            var user = User.Identity.GetUserId();
            var getuser = db.AspNetUsers.Find(user);
            var match = db.Matches.Find(id);

            Random rnd = new Random();
            var peg = rnd.Next(match.PegRangeMin, match.PegRangeMax);

            var alreadyExists = db.Pegs.Any(x => x.MatchId == match.Id && x.PegNo == peg);

            if (alreadyExists == true)
            {
                //do somthing
            }

            var result = new Booked()
            {

                MatchId = id,
                BookerId = user,
                TicketNo = guid.ToString(),
                //pegNo = randompeg
            };

            db.Bookeds.Add(result);
            db.SaveChanges();

            if (result == null)
            {
                throw new Exception();
            }

            var getmatch = db.Matches.Find(id);

                var body = "<p>Email From: {0} ({1}) </p>" +
                           "<h3>Match Booking Confirmation:</h3>" +
                           "<b>Ticket Number:</b><p> {2} </p>" +
                           "<b>Venue:</b><p> {3} </p>" +
                           "<b>date and time:</b><p>{4}</p>";

                var message = new MailMessage();
                message.To.Add(new MailAddress(getuser.Email));
                message.From = new MailAddress("nicholas.mciver@Activeplan.co.uk");
                message.Subject = "Matchbooker Confirmation Email";
                message.Body = string.Format(body, getuser.GetFullName(), getuser.Email, result.TicketNo, getmatch.Fishery.Name, getmatch.DateTime);
                message.IsBodyHtml = true;

                using (var smtp = new SmtpClient())
                {
                    smtp.Send(message);
                }

            return Ok();
        }