multiple identical addEventListeners added multiple times?

Adding something like

x = document.getElementById("toolbar-menubar");
x.addEventListener("click", function(event) { 
alert(event.button+":"+this.id); }, false);

multiple times (Fx17, using something like ExecuteJS), say executing it 
4 times, will result in 4 alerts when the Fx menubar is clicked.

According to the doc[1] (and by longtime assumption), this shouldn't be 
true and all but the first listener are discarded.  Yet I get 4 alerts. 
  Seems very bad..

[1] https://developer.mozilla.org/en-US/docs/DOM/element.addEventListener