Why does this simple javascript execute multiple times?
Behaviors will be executed on every request, including AJAX requests.

you can use the once function to make sure it is only triggered once. 

(function ($, Drupal) {
  Drupal.behaviors.mei_library_turn = {
    attach: function(context, settings) {

       $('main', context).once('mei_library_turn').each(function () {
          console.log('ready1');
       };)


      // There would be a lot more code in here, but I've stripped it all out to debug.

    }
  };

})(jQuery, Drupal, drupalSettings);