fix a bug when use a event delegation

<div class="de">
<div class="a">a</div>
<div class="b">b</div>
</div>

$('.de').on('tap', '.a', function(){
$('.log').append('.a <br>');
});

$('.b').on('tap', function(){
$('.log').append('.b <br>');
});

this situation .b will trigger twice.
but if you use tap and click both, it still will hava problem