When user flags or votes to close as a duplicate, a "Possible duplicate of [question]" comment is auto-generated on their behalf. When the closure succeeds, the comment is automatically deleted. But when the closure fails, the comment remains there as it is.
The comment is still useful in the sense that it establishes a link between the two questions (so they appear in the right sidebar). They might not exactly be duplicates, but they are definitely related to each other.
However, because the comment was added automatically as part of the closure, it certainly is no longer needed once the closure is over. It should be deleted, or better, edited into "Related" form.
Thus the suggestion is that the comment of the form
"Possible duplicate of [link]()" should be automatically edited into
"Related: [link]()" once the dupe closure for that particular
linked question fails.
Addendum: IMO, saying that a question is a "possible duplicate" of another question is not the same as saying they both are "related". There can always be two distinct questions about the same closely related topic (like this one). A failed duplicate closure indicates that the questions were so closely related to each other that they were mistaken as a duplicate.
Once the dupe closure has failed, the "possible duplicate" comment makes little sense, and therefore should be edited into "Related".