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I agree with Felipe. When a mathematician resubmits a paper to a different journal, she is soliciting a second opinion, as I think is her right. If there was a clear, serious error that you found the first time you refereed a paper and you still see it in the second version, then I think it would be helpful to communicate that to the editor. But I would guess that's a relatively rare situation: most papers are rejected because the referee just doesn't feel they are up to the journal's snuff. Get someone else to decide this the second time.