circular mentions are Webmentions for pages that mention another page because that other page mentions the first page.
Alice writes a post. Bob writes a comment in reply to Alice, and sends Alice a Webmention. Alice's site receives the webmention and shows Bob's reply as a comment underneath her post. By doing this, Alice's page now also has a link to Bob's page. Someone could then send Bob a Webmention, and it would be a valid Webmention. This would be a circular mention.
How to prevent?