Hur kollar man om email är upptagen i databasen innan registrering ASP.NET MVC?

Violation of PRIMARY KEY constraint 'PrimaryKey_a4dbd519-6b67-4fa6-8afe-08f838e4f6b5'. 
Cannot insert duplicate key in object 'dbo.User'. The duplicate key value is (test@hotmail.com).
The statement has been terminated.

 [HttpPost]
        public ActionResult Registration(LoginLogout.Models.User user)
        {
            if (ModelState.IsValid)
            {
                using (db)
                {                    
                    var crypto = new SimpleCrypto.PBKDF2();
                    var encrypPass = crypto.Compute(user.Password);
                    var sysUser = db.Users.Create();

                    sysUser.Email = user.Email;
                    sysUser.Password = encrypPass;
                    sysUser.PasswordSalt = crypto.Salt;
                    sysUser.UserID = Guid.NewGuid();

                    db.Users.Add(sysUser);
                    db.SaveChanges();

                    return RedirectToAction("Index", "User"); 
                 
                }
            }
            else
            {
                ModelState.AddModelError("", "Inloggningsinformationen är fel.");
            }
            return View();
        }