If, as a reviewer, you are going to tell an author that 'work has a;ready been done on this topic', which in my opinion editors see as a killing blow, please include the citations to such work so the authors can follow-up on it. Otherwise, such claims could be used...unethically.
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Also, if you as a reviewer claim that work has already been done on this topic without bothering to let author and editor know *which* work that was by providing a citation, you are a dick.
One time an editor sent me a paper to review, and it turned out I had already published a near identical paper the authors didn't know about. I thanked the authors for their independent replication and passed it with minor revisions.
