I am sorry to learn about the journal’s decision on your manuscript.
A redundant publication is also referred to as self-plagiarism and is defined as a type of plagiarism in which the writer republishes a work in its entirety or reuses portions of a previously written text while authoring a new work. It is acceptable for authors to re-use their copyrighted works if the previously published work is appropriately cited. However, it is considered unethical if the authors portray a previously published work as new, when it is not.
A redundant publication could also lead to copyright infringement. Once a manuscript is accepted for publication, the authors are often required to submit a signed copyright form to the publishers, wherein the author guarantees that the work is original and that no part of it has been previously published. Though authors can quote from their previous publications with the relevant citations, duplicate publication of previously reported data is considered unethical.
According to guidelines issued by the Committee on Publication Ethics (COPE,) journal editors should consider retracting a paper if it is found to be redundant. To prevent submission of redundant manuscripts, individual journals follow specific guidelines. For example, the New England Journal of Medicine insists that authors should submit to the editor copies of any published papers or other manuscripts in preparation or submitted elsewhere that are related to the manuscript to be considered by the journal.
In the case of your manuscript, which is based on one of your earlier published studies, it is important that you provide citations for all references to your previous work. Further, to avoid being considered guilty of self-plagiarism, it would be advisable to add a note to the editor in the cover letter, informing him/her of the previously published study that forms the basis of the current study.
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