Competition problem in java web application

Good evening guys,

See the following code:

Controller

@WebServlet("/testConcurrency")
public class TestConcurrency extends HttpServlet {
    private static final long serialVersionUID = -6124392524678396101L;

    @EJB(name="bs/UsuarioBS/local")
    private UsuarioBSLocal usuarioBS;

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String emailStr = request.getParameter("email");

        System.out.println("[Inicio] usuarioBS.testConcurrency();");

        try {
            usuarioBS.testConcurrency(emailStr);
        } catch (BusinessException e) {
            e.printStackTrace();
        }

        System.out.println("[Fim] usuarioBS.testConcurrency();");
    }
}

BS

@Stateless(name="business/UsuarioBS")
public class UsuarioBS implements UsuarioBSLocal {
    private Logger log = Logger.getLogger(UsuarioBS.class);

    @EJB(name="dao/EmailDAO/local")
    private EmailDAOLocal emailDAO;

    @Override
    public void testConcurrency(String emailStr) throws BusinessException {
        try {
            boolean existeEmail = emailDAO.existeEmail(emailStr);

            System.out.println("Existe email ["+ emailStr +"]? " + existeEmail);

            if (!existeEmail) {
                Email email = new Email(emailStr);
                email.setPessoa(new Pessoa(1l));
                email.setTipoEmail(TipoEmail.PRINCIPAL);

                try {
                    System.out.println("Waiting...");
                    Thread.sleep(5000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                emailDAO.inserir(email);

                System.out.println("Email ["+ emailStr +"] inserido!");
            }
        } catch (DAOException e) {
            e.printStackTrace();
        }
    }
}

If I double-click the /[email protected] url in less than 5 seconds it will insert the email twice, which can not occur.

Ps: Thread.sleep () is only to illustrate the problem. In other places of my application I also call the emailDA.existeEmail () method.

How can I resolve this?

Thank you very much.