colinwalker duplicate ping error

Pinging URL #2112: https://colinwalker.blog/this-time-feels-different/
Error: 100: Webmention answer HTTP status code is not 2xx but 500
Pingback response: Status code 500, headers: Array
(
    [date] => Thu, 22 Mar 2018 11:20:02 GMT
    [server] => Apache
    [expires] => Thu, 19 Nov 1981 08:52:00 GMT
    [cache-control] => no-store, no-cache, must-revalidate
    [pragma] => no-cache
    [x-robots-tag] => noindex
    [link] => <https://colinwalker.blog/wp-json/>; rel="https://api.w.org/"
    [x-content-type-options] => nosniff
    [access-control-expose-headers] => X-WP-Total, X-WP-TotalPages
    [access-control-allow-headers] => Authorization, Content-Type
    [allow] => POST, GET
    [connection] => close
    [transfer-encoding] => chunked
    [content-type] => application/json; charset=UTF-8
)
 
HTTP body: {"errors":{"comment_duplicate":["Duplicate comment detected; it looks as though you&#8217;ve already said that!"]},"error_data":{"comment_duplicate":409}}