Avoid multiple event listeners from .focus
<p>I currently have a textbox that I am invoking a keyboard stroke on focus:</p>

<pre><code>$myTextBox.on('focus', function(e){
    $(document).keydown(function(e){ 
       if(e.which==13)
         e.preventDefault() 
    });
    $(document).keyup(function(e){
       if(e.which ==13)
         alert("hey");
    });
});
</code></pre>

<p>If I click on this multiple times pressing 'enter' once will cause many alerts, how can I avoid this so that only it is only invoked once.  </p>
