I have just submitted my review of a manuscript to a journal, which allowed me to see other reviewers' comments once I submitted mine. I have points of disagreement with this other reviewer, but I am sure this is normal. What alarmed me was that, his/her review comments ended with a suggestion for the authors to cite additional references which, in my opinion, are not directly relevant to the manuscript, but were suggested on the pretense that they were published in the same journal, which is weird. These references have something in common in them: they are authored by the same group of people. Although I cannot be sure that this reviewer is one of them, I have the feeling that h/she may be, and that this practice may be common. In the past, when I was the author, a reviewer also suggested me to include additional references, but these references were not authored by the same people, and anyway I ended up not including them as I thought they were not directly relevant. But as a reviewer, what should I do in this case?
I would recommend raising your concern to the editor. If the editor is honest, they will not ask the authors to follow the citations en masse.
Given that the citations are all in the same journal, however, it is possible that the editor is part of the same citation cartel and may blow off your concern. In this case, it may be worth raising the concern to the publisher and/or one of the major indices. Citation cartels have been gaining attention in recent years, and if this is part of a systematic pattern of coercive citation, then those larger organizations may be motivated to investigate and sanction.
My answer depends on where in the review process the manuscript is.
If you believe the manuscript will come back to you for a second round of reviews, I would wait until then. At that point, see how the authors handled the issue. If they included irrelevant citations, tell them in your review. Something along the lines of:
I would then add a confidential note to the editor saying that they may want to comment on if the cited work is needed.
If you believe the manuscript will not come back to you (either because the authors will give up or it will be accepted), then you may want to tell the editor directly. I tend to defer to the editor on these issues, so would not say anything.