HTML5 Javascript Button click triggers multiple times

There is a button when I click once, it increments the qtitlebuttonNo by one. But now out of no reason, it jumps from 1 to 3. But after 3 are all functioning well. Just don't know why when I click the first time, qtitlebuttonNo++; triggers twice so it increments by two from 1 to 3. What can I do to solve it? Thanks.

game2f2 = this;

game2f2.confirm_btn_qanswer.addEventListener("click", showNext_qanswer.bind(game2f2));

var clickedOnce = false;

function showNext_qanswer(){

if(!clickedOnce){

console.log("game2f2 qtitlebuttonNo",qtitlebuttonNo);

game2f1.gotoAndStop(0);

if(qtitlebuttonNo<15) {

qtitlebuttonNo++;

}

else gameOver = true;

game2f1.qno.gotoAndStop(qtitlebuttonNo);

game2f1.qtitlebutton.gotoAndStop(qnumbers[qtitlebuttonNo]);

clickedOnce = true;

}

}