The keypress event when handles without proper event handler then causes duplicate execution here is how you can fix that

// this will allow duplicates if I press enter really fast twice.
$("#messageBox").keypress(function(e) {
//alert(e.keyCode);
if(e.keyCode == 13) {
chatApp.sendMessage();
}
});

As a workaround so I can use JQuery selectors, I just get the raw DOM element instead and then attach the event to the raw DOM element by using the JS array notation:

// use Jquery to get the element, then attach event to the RAW DOM element
$('#messageBox')[0].onkeypress = function(e) {
var key = e.which||e.keyCode;
if(key == 13)
{
chatApp.sendMessage();
}
};