That’s where some clever CSS can come in handy:
<style media="screen">
.updated[datetime="{{published_at}}"] { display: none; }
</style>
<time class="updated" datetime="{{updated_at}}">Updated: {{updated_at}}</time>
<time class="published" datetime="{{published_at}}">Published: {{published_at}}</time>
The CSS here is selecting the updated date element with an attribute value of the published date and hiding it, it’ll only appear if the updated date is different to the published date