Duplicated H1/H2/H3s

might be related to something I'm doing in
✨ Book TOC (Table of Contents) RTBC

This was a theming issue.

I resolved by copying this twig from the book module:
book-node-export-html.html.twig

to my theme templates/ folder

and then overriding the twig, removing the H1 entry.

Then using theme debug, I created this twig from the suggestions

node--book--print.html.twig

and made my changes.

actually, could add a little snippet about this in the README

To get more control of the PDF generated html start by enabling your theme debug in your sites/default/services.yml

Add this code:

echo print_r($bookHtml, TRUE);
die;

in modules/contrib/book_pdf/src/EventSubscriber/BookPdfPreGenerateSubscriber.php
just after $bookHtml is instantiated in the code.
Rebuild caches

You'll have to search for theme debug in the rendered output

Mine was
node.html.twig comming from a parent theme

There's several theming suggestions.
One of them is : node--book--print.html.twig

I modified the twig as desired.

There is another twig also that I modified, this one is provided by core.

core/modules/book/templates/book-node-export-html.html.twig

There were no theme suggestions available, however I simply copied this twig to my custom theme templates/book folder.

Then I modified this twig, in my case, I removed the H1 in this twig.

Adjust your twigs as desired.

https://git.drupalcode.org/project/book_pdf/-/merge_requests/7.patch

https://git.drupalcode.org/project/book_pdf/-/merge_requests/7.patch

This README.md is ready