How to use an HTML template more than once [duplicate]

I created a bit of HTML in a template tag that I want to use from time to time in my webpage. I can get it to display once no problem, but if I try to add it elsewhere in the page it does not show up. I have included a much simplified version of the code below. Can someone enlighten me as to what I’m doing wrong?

var template = document.getElementById('testtemplate').content;

document.getElementById('firstDiv').appendChild(template);

document.getElementById('secondDiv').appendChild(template);
<template id='testtemplate'>
    <div class = calendarcontainer>
        <h1>hello</h1>
    </div>
</template>

<div id='firstDiv'></div>
<div id='secondDiv'></div>