Can I tell the authors I was a referee for the same paper for another journal?
I suggest copy-and-pasting your entire review, after double-checking nothing has changed, i.e., your review is still current. You should make your position clear to the editor and I suggest opening with a statement such as: I reviewed this manuscript a week ago for another journal. The authors have not addressed my comments and my opinion remains unchanged: ... Unlike another answer, I see no problem stating you were the reviewer to both the editor and the authors, since it'll be obvious to the authors anyhow.