Avoid Adding Event Multiple Times

Either don't use a new function each time, or use a class or something to tell you you've added it.

Not using a new function each time

MooTools' addEvent is a fairly thin wrapper on addEventListener/attachEvent, which won't add the same function twice. So if you ensure you're using the same function, you can call addEvent again without it doing anything:

// Somewhere it's created **once**
function keyDownHandler() {
    // ....
}

then:

element.addEvent("keydown", keyDownHandler);    // Adds it only if not there

Live Example:

addIt();
addIt();

function addIt() {
  $("foo").addEvent("click", handler);
}

function handler() {
  var p = document.createElement('p');
  p.innerHTML = new Date();
  document.body.appendChild(p);
}

<div id="foo">Click me</div>
<script src="https://ajax.googleapis.com/ajax/libs/mootools/1.5.0/mootools-yui-compressed.js"></script>

Remembering you've added it:

if (!element.hasClass("keydown-handler")) {
    element.addClass("keydown-handler").addEvent("keydown", function() { /*...*/});
}

Live Example:

addIt();
addIt();

function addIt() {
  var element = $("foo");
  if (!element.hasClass("click-handler")) {
    element.addClass("click-handler").addEvent(
      "click",
      function() {
        var p = document.createElement('p');
        p.innerHTML = new Date();
        document.body.appendChild(p);
      }
    );
  }
}

<div id="foo">Click me</div>
<script src="https://ajax.googleapis.com/ajax/libs/mootools/1.5.0/mootools-yui-compressed.js"></script>

Maybe your problem is not where or when to add event handler, but how to add. It is possible to delegate events to parent element. For example, this is a classic way of adding an event handler directly to the target element.

new Element( 'p', { 'text': 'Some Text' } ).inject( $( 'container' ) );

$$( '#container p' ).addEvent( 'click', function() {
    console.log( this );            
});

new Element( 'p', { 'text': 'Some Text' } ).inject( $( 'container' ) );

And, of course, only the first paragraph has click event.

And below is an example of event delegation.

$( 'container' ).addEvent( 'click:relay(p)', function() {
    console.log( this );            
});             

$( 'container' ).adopt([
    new Element( 'p', { 'text': 'Some Text' } ),
    new Element( 'p', { 'text': 'Some Text' } )
]); 

Since event listener is assigned to a parent for all of its children, all paragraphs will have the click event, no matter when and where they are created. It is very useful when you are adding elements into the page dynamically.

I do not know how much this helps, but it's good to know.

Element.Delegation