Unlike some blogging platforms, WordPress sends self-pingbacks when a blogger makes internal links.
Aside from the possible effect this might have on SEO, its annoying to be notified by the Akismet plugin every time one links to their previous posts.
Apparently there are plugins to deal with this, but a simple solution is to use a relative URL when self-linking.
So, for example, if I was making an internal link to this post I would not use this absolute URL:
https://www.chou-seh-fu.com/wordpress/how-to-stop-self-pingbacks/
I would use this relative URL instead:
/wordpress/how-to-stop-self-pingbacks/