transitionend event fires twice

transitionend fires for each property transitioned, in your case top and left.

You can access the property associated with the event at event.propertyName.

There’s no “transitionsend” event, so you will probably need some hackiness such as filtering the transitionend callback handling for only one of the transitioned properties. E.g.:

function (event) {
    if (event.propertyName == 'top') {
        //put your code here
    }
});

ps. No browser fires the MSTransitionEnd event. It was at some point in the MS docs, but sometime before the IE10 beta release it was replaced by the standard transitionend event.