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?
You could simply go for the half-measure: tell the colleague you are one of the reviewers, but not which one. If they are reasonable, they will be able to understand your position, and will not dig into the issue anymore. If they're not reasonable, well...why would you care then, i suppose.