code: public boolean add(ConfigurationSection section) {
      if (section instanceof SingletonConfigurationSection) {
        if (getSection(section.getClass()) !=  null) {
          throw new IllegalArgumentException("Section of same type already inserted: " + section.getClass().getName());
        }
      }
      return sections.add(section);
    }
docs: Add a new configuration section to the container.
  
  @throws IllegalArgumentException if same type is already present and a singleton
  @return always {@code true}