fix: 修复重复添加事件判断

[lfb]

当多次添加同个事件，会触发多次

var fn = function(){
    console.log('恭喜您已经阅读完毕')
}
progressBar.once("end", fn)
progressBar.once("end", fn)
progressBar.once("end", fn)
progressBar.once("end", fn)