circular mentions

 circular mentions  are Webmentions for pages that mention another page because that other page mentions the first page.

Example
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?
One way to prevent this is to use js;dr to your advantage and load previews of received Webmentions / comments via Javascript. This way they do not show up in the HTML as received by curl / crawlers / Webmention validators.