When cloning an element that has mouseenter or mouseleave events, the clone will receive a duplicate mouseover and mouseout event causing the handler to double-fire.

So if the jQuery.cache shows this for the original:

mouseenter: Array(1)

mouseover: Array(1)

mouseleave: Array(1)

mouseout: Array(1)

the clone will have:

mouseenter: Array(1)

mouseover: Array(2)

mouseleave: Array(1)

mouseout: Array(2)

Test: http://jsfiddle.net/9y7tp/

The hover for the original element will toggleClass the "blue" class properly and show one alert.

The hover for the clone double-fires the handler, so the toggleClass is called again, and you get 2 alerts.

var $hoverbox = $('.hover').hover(function(e) {

$('#box').toggleClass('blue');

alert(e.type);

});

$hoverbox.clone(true).text("Hover clone").appendTo('body');