.cs

public class Program{
  public void CreateUser(string username, string firstname, string familyname){
    var user = new User(
      new UserName(username),
      new FullName(firstname, familyname)
    );
    var userRepository = new InMemoryUserRepository();
    var userService = new UserService(userRepository);
    if(userService.IsDuplicated(user)){
      throw new Exception("重複しています");
    }else{
      userRepository.Save(user);
    }
  }
}

public class UserService{
  private readonly InMemoryUserRepository userRepository;
  
  public UserService(InMemoryUserRepository userRepository){
    this.userRepository = userRepository;
  }

  public bool IsDuplicated(User user) {
    var name = user.UserName;
    var searched = userRepository.Find(name);

    return searched != null;
  }
}