What to do if a colleague is reviewing an unchanged paper that has been rejected before on my recommendation?
If you were asked to re-review an "unchanged" manuscript there are a number of things you can do (e.g., Asked again to review a paper, when the authors don&#39;t wish to modify it). The issue is that this is not the case. You are no longer part of the review process.

The first thing you should do is STOP. The behavior you have engaged in so far has been completely unethical and a clear violation of every reviewer agreement I have ever seen.


Your colleague should never have told you about the paper under review.
You should not have mention that you reviewed a similar paper in the past.
Neither of you should have mentioned authors or the title.
The actual manuscripts should never have been shared and/or compared


To a lesser extent, it is not even clear why you still have your copy of the manuscript.

The Committee on Publication Ethics (COPE) provides Ethical Guidelines for Peer Reviewers which can be thought of as best practice. These include:


respect the confidentiality of peer review and not reveal any details of a manuscript or its 
review, during or after the peer-review process, beyond those that are released by the 
journal
not involve anyone else in the review of a manuscript, including junior researchers they are 
mentoring, without first obtaining permission from the journal; the names of any individuals 
who have helped them with the review should be included with the returned review so that 
they are associated with the manuscript in the journal’s records and can also receive due 
credit for their efforts. 
keep all manuscript and review details confidential.