CQRS applying command handler business logic

I am apllying sample CQSR to an application. And I am confused about applying logical rules.

public class UserCommandHandler : ICommandHandler<CreateUserCommand>
{
    private readonly IUserRepository repository;

    public UserCommandHandler(IUserRepository repository)
    {
        this.repository = repository;
    }

    public void Handle(CreateUserCommand command)
    {
        User user = new User();
        user.Username = command.Username;
        user.Password = command.Password;
        user.Email = command.Email;
        user.CreteDate = DateTime.Now;
        // Rules
        var isUserDuplicated = repository.GetAll().Any(u => u.Username == command.Username);
        var isEmailDuplicated = repository.GetAll().Any(u => u.Email == command.Email);

        repository.Add(user);
        repository.Save();
    }
}

1- Where can I apply business rules to chack if username duplicated or email duplicated.

2- My command handler is void. So how can I send duplication notification to users?