Is it generally safe to add same Drupal javascript multiple times?
Usually, you add javascript with Drupal's behaviors as explain in the doc. If you have AJAX callbaks, javascript behaviors will be reloaded and this can cause some trouble.

For exemple, on my website I added moving clouds in the header with a javascript behavior, the speed beeing added by increments, each time I loaded an AJAX form it was sped up unintentionally. I could either add a jQuery Once behavior or use the common $(document).ready function.

So in conclusion, it depends on the way you call the code inside your javascript file. Your javascript code called inside a $(document).ready function should be loadded once, even with AJAX callbacks. On the contrary, a Drupal javascript behavior will be executed each time your DOM is updated, exept with jQuery Once enabled:

Drupal.behaviors.mybehavior = {
  attach: function (context, settings) {
    $('input.mybehavior', context).once('mybehavior', function () {
      // Apply the MyBehaviour effect to the elements only once.
    });
  }
};


Now if you are concerned with others javascript coding standards, see the official Javascript coding standards.