Yesterday I flagged this question: restriction diagonalizable operator is diagonalizable as a duplicate of this other question: Diagonalizable transformation restricted to an invariant subspace is diagonalizable.

I did not add a comment to the original post, but someone (a moderator presumably) inserted a comment in my name mentioning the second question to the OP. And the question was not marked as duplicate.

I am curious if that's the way things are done nowadays? It seems clearer to me to mark the post as duplicate instead of just adding a comment.

For the long term maintainability of the site, more that even marking a question as a duplicate, it seems to me it would be helpful to even delete completely questions that are duplicates and that don't add any value to what has already been answered comprehensively in other questions. (The current question did not have any answer.) But that may be a separate discussion.

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    $\begingroup$ This is an automated process, when the post is eventually closed as duplicate, your comment would disappear. $\endgroup$ Apr 29 at 20:33
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    $\begingroup$ The purpose of the automated comment (triggered by the first vote to close as a dupe) is to (1) Notify the asker of the question; (2) Alert other users to a potential duplication of another question. By the way, thanks for noticing the duplication! $\endgroup$
    – amWhy
    Apr 29 at 20:59
  • $\begingroup$ "the question was not marked as duplicate." One vote to close as a duplicate is not enough to mark a question as a duplicate (unless the one voter is a moderator), so you shouldn't be surprised that the question was not marked as a duplicate. It will be so marked, when/as/if four more voters agree with your claim. $\endgroup$ Apr 30 at 6:05

In the past, when a question was marked as a duplicate, the body of the question was automatically edited to insert a link to the duplicate question. Unfortunately, this leaves meta commentary in the body of the question. Meta comments don't belong in questions (or answers) and so this behaviour was changed (quite a while ago---the change was before my time here).

Instead, when someone votes to close a question as a duplicate, a comment of the form

Does this answer your question? [link to duplicate target]

is automatically added to the comments below the question. The comment is attributed to the first person to vote for that duplicate target. Subsequent duplicate votes give a +1 to the comment. When and if the question is closed as a duplicate, the comment is automatically deleted, but a close banner is added to the top of the question.

Regarding the eventual deletion of duplicate questions, the design philosophy of the SE sites is that duplicates should generally be preserved, as they are meant to act as signposts for future askers. If you don't think that a question is worthwhile as a signpost, it might be better to (a) provide a link to the duplicate question in the comments, and (b) vote to close the question for some other reason (e.g. for lacking context).

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    $\begingroup$ If you don't like the wording of the Comment posted over your name, you can edit it (within the appropriate time frame) or delete it entirely (as you can with any of your own comments). However these automated Comments do have a value for other Readers who may see the duplicate proposed is not optimal or want to propose an alternative before the close vote is finalized. $\endgroup$
    – hardmath
    Apr 29 at 23:07
  • $\begingroup$ Unfortunately, one has to edit the comment if the title of the duplicate target contains MathJax with “less than” or “greater than” symbols, due to this bug math.meta.stackexchange.com/q/29632, which still has not been fixed. $\endgroup$
    – Martin R
    Apr 30 at 7:47

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