Avoid reattaching javascript when a form is rebuilt by ajax callback
As Clive mentioned, the usual way to solve this is to use once() when attaching behaviors:

Drupal.behaviors.myBehavior = {
  attach: function (context, serverSettings) {
    $('elementSelector', context).once('myId', function () {
      // The code that will be executed only once.
    });
  }
};


Also, check the official docs on Managing JavaScript in Drupal 7.