“Often called “self-plagiarism,” text recycling occurs frequently in scientific writing. Over the past decade, increasing numbers of scientific journals have begun using plagiarism detection software to screen submitted manuscripts. As a result, large numbers of cases of text recycling are being identified, yet there is no consensus on what constitutes ethically acceptable practice. Text recycling is thus an increasingly important and controversial ethical issue in scientific communication.” (Texrecycling.org, retrieved Sept 6, 2019.
“Text recycling is the reuse of textual material (prose or visuals) from one document in a new document where (1) the material in the new document is identical to that of the source or substantively equivalent in both form and content; (2) the material serves the same rhetorical function in both documents; and (3) at least one author of the new document is also an author of the prior document (unless the source contains a statement explicitly granting permission for its contents to be recycled).” (Moskovitz, 2018).
Some text recycling, e.g. in the introduction and methods section may be judged both inevitable and acceptable or even desired (Moskovitz, 2016), but there seem to be different "rules" in different disciplines.
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