This usually doesn't happen, as the entities with existing id will be only merged rather than creating the new instance. Try checking your equals and hashcode implementations for 'Articulo' class, as that plays a central role in merge mechanism.
If you still want a workaround,
public void persistArticulo(Articulo articulo)
{
try {
EntityManager entityManager = entityManager();
entityManager.getTransaction().begin();
// else persist
entityManager.persist(articulo); entityManager.getTransaction().commit(); entityManager.close(); return ; }