I serve as an associate editor at journal X and recently handled a paper, which I finally rejected after multiple rounds, mainly because of the points raised by a reviewer R. R's points were quite pertinent, and addressing them would have meant a rather thorough rewriting of the paper. The authors chose not to do so but to defend their current paper. Reasonable people could disagree here, and I decided to reject, which was not an easy decision, because the paper was very interesting otherwise.
R indicated that they did not want to re-review yet another revision.
A few days ago, a different journal Y sent me an invitation to review this exact paper. (It's single-blind, so I see that the authors and the title are identical, so it is the paper.) I'm a bit torn, because R's points are relevant, but the paper is interesting otherwise.
I don't know yet whether the authors have addressed R's points in the new submission, because I can so far only access the abstract.
I currently lean towards accepting the invitation to review, and include R's points in my assessment of the current version. However, even if I don't state in the review that I got those points from R (I would definitely tell the editor), it will be quite obvious that I either am R, or at least have seen R's review for X. I still believe this procedure makes sense, because I believe it will increase the chances for a fundamentally good paper to be published, in an even better shape than it (likely) is now. But this of course is ethically iffy: it interferes with review confidentiality, and by not explicitly acknowledging that I got these points from R, I would essentially plagiarize their review.
So should I explicitly acknowledge that I got some of my points from an anonymous review of an earlier version that I saw? Or should I take the easy way out and decline the review invitation?