Let’s say you have already published material in another journal. You may try to use some of your past material in a new work that you are submitting. You might even directly copy-and-paste your own text. It is wrong to do this. Copying your own previously published material or data in a new piece of work is known as text recycling, redundant publication, or “self-plagiarism”. Even if the work is your own and you are not copying the ideas from someone else, self-plagiarism is still unethical.
Reusing or republishing the same work, either in whole or in part, is unethical because:
If you want to use any of your previously published work, you must paraphrase any material you use and provide a citation for it. Even if you have kept the copyright on your work (for example, as an open access article), you must still paraphrase your own words. Never reuse your own previously published material or data in a new work without paraphrasing it and citing the original source — even if you are that original source!