What is the best practice to find out if the manuscript that you are reviewing is a duplicate article? I have tried searching the literature, using plagiarism checker but nothing came up and still the manuscript looks so familiar!!
A Google search for brief phrases is often an effective technique. Often plagiarists change every fifth, sixth, or tenth word to try to sneak a stolen text through the filters.
Sometimes, a journal article is republished as a chapter in a book published as an anthology of previously published journal articles, which, of course, includes a posting of all of the pertinent publication information for the respective journal articles included in the book.
Why is this? Journal articles in a single issue often include articles on different topics which are selected for publication in a journal that is devoted to a general scientific topic. In order to provide coverage for the numerous sub-topics within the generalized area covered by the journal, any given issue of the journal usually covers different sub-topics. Subsequently, a specialist who works in one of the numerous sub-topics, in the course of his or her research, discovers a number of top notch articles that have been published in various different journals. Then he or she finds a book publisher who is interested in the sub-topic and who finds that there might be a readership for the sub-topic, and finally, decides to accept the manuscript of journal articles gathered by the prospective book editor (specialist in the sub-topic who found the articles) for publication.
The above outlined scenario may provide you with one answer to your question, and it has nothing whatsoever to do with anything illegal or involve plagiarism !
Therefore, this type of a situation may be something for you to consider: perhaps you saw a journal article which, subsequently, was published as a chapter in a book !