Somebody has just pointed this out, which might help fix this issue:

Jetpack’s lazy loading does actually print an extra avatar, but it’s placed between <noscript> tags. If your theme than uses wp_kses_post to escape scripts, it apparently also removes the <noscript> tags, hence making a second author image visible immediately after the original one.

https://wordpress.org/support/topic/author-image-signature-displayed-twice-after-a-post/#post-12096318

Thank you, that's what I needed to know. This can be closed.

Does PenNews have its own Lazy load or does that option disable the Lazy load used by other plugins?

The avatar shows two images when using Jetpack's Lazy load but works properly with other plugins', I already explained.

I couldn't understand your first paragraph, sorry. Does PenNews have its own "Lazy load"?

It shows one image only because I'm using a different plugin now, as I said.

This might be a problem with Jetpack, seeing how "lazy load" seems to work correctly when using other plugins, so I'll just be using those instead.

I've found a "Disable Lazyload" option enabled by default in Customize -> General Options. I don't quite understand why such an option exists.

Anyway, I disabled it but the avatar still shows duplicated.

The point of "lazy load" is not to load the images until they appear on screen, and the theme is loading it twice instead, defeating the point.

Hiding one of them after been loaded is not a fix to the problem, the image needs to just be loaded once.

The author avatar below the post contents is appearing twice when Jetpack's "Lazy load images" feature is active.

Is there a way to fix this?