It is an automatic comment rather than a statement.
From How do comments work?
What are automatic comments?
In several cases, the system will automatically add comments to posts on your behalf. You still own the comment, and it will appear the same as if you had manually posted the comment. You may delete the comment if you desire, but this is discouraged as it is helpful to the original poster: the automatic comments provide information on how to improve the post or why it is not appropriate.
When someone flags or votes to close a question as an exact duplicate, the system automatically posts a comment to that question which reads "possible duplicate of -link-", as long as no such link has already been posted in a comment. The comment will be automatically deleted if the question is closed as a duplicate.
This mechanism had feature requested as Autocomment when voting to close as duplicate.
Now, talking to the point, comments are considered less important than posts and statements.
Well, we've already polite banner to the questioner when question get duplicate close vote:
The question may have already an answer here:
[Title of question](link to the question)
Also, the close statement after question get closed is also fine:
This question was marked as an exact duplicate of an existing question
Anyway, the owner of comment is free to edit or delete the comment.