As the center of gravity for scientific research is moving from old locales with longstanding traditions to new geographies with forgotten traditions or which never had them, plagiarism has become a major issue. At times it feels like being in the Wild West.
Sometimes papers are pure copies of previous work. More often, authors have not yet developed a proper hygiene for citing related work. I am always surprised how often authors cite secondary references instead of primary references, a fact I tend to take as laziness and punish with a negative review.
In Volume 6, Issue 1 of the SPIE Journal of Nanophotonics, Editor-in-Chief Akhlesh Lakhtakia has written a very useful editorial on this topic. Follow this link: http://dx.doi.org/10.1117/1.JNP.6.069901.