Interesting comment from AnonNeuro:
Reviews are confidential, so I don't think you can share that information. Saying "I'll review it again" is the same as saying "I have insider knowledge that this paper was rejected elsewhere". Better to decline the review due to conflict.
I don't think I've ever followed this as a rule. I have definitely told editors when the manuscript has not been revised from a previously critiqued version in the past (I don't say which journal had rejected the authors' work). But I can't say that I invariably mention it either. If the manuscript had been revised somewhat, why bother. If I like it and want to see it published, mentioning I've seen a prior version elsewhere seems counterproductive.
This comment had me pondering my lack of a clear policy.
Maybe we should tell the editor upon accepting the review assignment so that they can decide if they still want our input?