Earlier I linked to multiple comments on the same post which produced multiple webmentions and, consequently, multiple instances of the "Related Post" listing in the comments.

Not ideal.

When displaying the /directory I cycle through the site comments and add the name of each author to an array. This allows me to check for an existing instance of that person and only display the first.

I have now replicated this check for self-webmentions using the source URL rather than comment author so a related post will be listed just once.