Colleague asks for feedback on a paper of which I am a reviewer More than once, I have found myself with a curious ethical dilemma.  Due to the nature of the dilemma, however, I have waited quite some time to ask this question, in order to increase the anonymity of those involved.

Here is the dilemma: a colleague of mine sent me a copy of a paper currently under review, saying they thought it would be of interest and wanted to get my feedback, since it is a subject on which I am known to have expertise.  The only problem was that I was one of the reviewers for the journal (and indeed had already turned in my review before my colleague approached me).

There was no ethical conflict for me performing the review, and no impropriety on the part of my colleague for sharing the manuscript either: we have similar research interests but have never collaborated closely, and the circumstances of the sharing mean there is no reason to think this could be an attempt to influence a possible reviewer.  Still, I found it challenging to interact in such a way that it would not breach my anonymity as a reviewer.

What do you think is the best way to handle such a situation?
Have you considered writing a fresh answer to the colleague without caring that you wrote (or are writing) a review? What I mean is, just write the answer on your knowledge of the paper without caring that you are also writing a review. I think this is a natural approach and does not really have a downside.

You know the paper quite in depth, and you can provide valuable feedback. You will do so (or already have done so) in a formal way through the peer-review system, but now you have the opportunity for formulate the feedback in a more casual way (compare it with writing down results for a formal publication in a journal or at a conference and a discussion of the results on your personal blog - you describe the same things in a different way of communication). The only downside I see is, that the person will get some more clues that you may be a reviewer but I am not sure if this really is a downside at all.

Another downside may be that it costs you more time and that you don't want to allocate time to provide feedback twice. In this case, you can politely answer something like "Thanks for the paper - I try to find time to read it soon and come back to you in case I have any feedback." I guess most people understand that this can also mean that they won't hear anything more from you on that matter…