Tiny JS code (jQuery, sorry guys) - that prevents double click on target forms.

prevent-double-click.js

// Prevent double click in some forms.
(function(jQuery) {
  "use strict";
  Drupal.behaviors.fixDoubleClick = {
    attach: function (context) {
      var k, selectors = [
        "#comment-form"
        // Add here forms with problems.
      ];
      for (k in selectors) {
        jQuery(context).find(selectors[k]).on("submit", function () {
          jQuery(context).find("input[type=submit]").attr({
            disabled: "disabled"
          });
        });
      }
    }
  };
}(jQuery));

Oh, and that's for Drupal, if you want it generic, replace the Drupal.behavior.fooBar using a nice jQuery(document).ready({ /* ... */ }); instead.