When I add <meta name="viewport" content="width=device-width, initial-scale=1.0" /> in the head, “tap” event is executed twice on andriod 5.1.1/chrome 49.0.2623.91.

dom.addEventListener("tap",function(){
    alert('will alert twice');
})

if I edit the code like blow will be OK:

dom.addEventListener("tap",function(event){
   event.preventDefault();
    alert('will alert once');
})

it was a tap's BUG or my fault?