Why does hovering over my image show another copy of my image, when in html made from rmarkdown with gitbook formatting?

[MBN]

I have a simple rmarkdown file (below) that creates an image at the start of the file, following a tip from here, after trying alternative methods here. I haven't found documentation of the tip.

I also use a yaml output file (_output.yml) with one line bookdown::gitbook: that creates a lovely sidebar index. However, including that makes output with odd behaviour - hovering near the image shows another copy of my image?

rmarkdown file code:

---
title: |
  ![](https://i.imgur.com/GiViTbA.png)  
  Look at that dog
output:
  html_document: default
---


# I'm a top level heading {-}

Note, you need to replace the image with a local image if you want to recreate the error in the rstudio viewer.
The error will be visible in the html file created when you knit, if you open in a browser connected to the internet.

```{r echo=FALSE, message=FALSE, warning=FALSE}
plot(cars)
```

Behaviour looks like this:

Looks even odder when published to RPubs, though i guess RPubs is not set up to take the gitbook format?