code: protected final void addPersistent(Class<?> entity) {
      if (entityClasses.contains(entity)) {
        addError("Entity class was added more than once: "
            + entity.getName()
            + "; to the same persistence module: "
            + getClass().getSimpleName()
            + (selector == null ? "" : " => @" + selector.getSimpleName()));
        return;
      }
  
      entityClasses.add(entity);
    }
docs: Adds an entity class for mapping into the current datastore. Each datastore has
  its own semantics for mapping fields of a class into its particular storage format.
  <p>
  For example, JpaModule maps fields into SQL columns whereas RedisModule converts
  an entire object into JSON for storage as a string. Customizing these semantics
  is specific to each datastore. Consult the specific implementations docs for details.