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.