WP_Widget_Factory::_register_widgets()

	public function _register_widgets() {
		global $wp_registered_widgets;
		$keys = array_keys($this->widgets);
		$registered = array_keys($wp_registered_widgets);
		$registered = array_map('_get_widget_id_base', $registered);

		foreach ( $keys as $key ) {
			// don't register new widget if old widget with the same id is already registered
			if ( in_array($this->widgets[$key]->id_base, $registered, true) ) {
				unset($this->widgets[$key]);
				continue;
			}

			$this->widgets[$key]->_register();
		}
	}