Nice work, @acaporrini! Thanks for contributing this nifty plugin. Is there any way you could envision adding a shortcode e.g. [toc] to generate a table of contents of headings and sub-headings within a post? This would make a long term dream a reality for me. 
I’m not sure this blue bookmark icon is really needed - it would be fine, in my view, for this to be a hidden feature that power users can use to deep link to sections within posts.
Typo? Looks like this URL should be
https://github.com/acaporrini/discourse-auto-bookmarks.git
I agree strongly with this. I would love to see this feature in core even without deduping anchors within posts. The benefits far outweigh the risks.