ActionScript 3.0 :: Event Listener Good Practices?

Anyone have any tips for good practice with using event listeners? I've gotten into the habit for checking for event listeners (hasEventListener()) prior to removing them, but no errors are thrown if you try to remove an event listener that does not exist, so should I even bother?

Also, I've often gotten into the habit of checking for a listener prior to adding it sometimes, just to make sure I'm not adding it twice (mostly due to slightly sloppy code when they're added/removed on an as-needed basis). But doing a quick test, it seems this is also not needed. It seems adding an event listener of the same type with the same handler twice overwrites the first:

Code:
stage.addEventListener(MouseEvent.MOUSE_DOWN, downHandler);
stage.addEventListener(MouseEvent.MOUSE_DOWN, downHandler);
// traces 'true'
trace(stage.hasEventListener(MouseEvent.MOUSE_DOWN));

[Code]...