Is it plagiarism to use your own work?

Plagiarism is claiming someone else's work or fragments of it as your own. It includes paraphrasing thoughts and phrases without referring to the source.
Self-plagiarism is plagiarism when an author uses fragments from a text of a previously completed work while creating a new scientific paper. He poses his existing material as a new paper and does not inform the reader that the material has already been published.
The American Psychological Association labels self-plagiarism as deception. The author misleads his audience. The effort put into composing the new work is negligible; thus, it is academic dishonesty.

It is considered self-plagiarism when an author:

• Republished a paper published in another edition without informing the publisher and readers.
• Split an extensive research paper into separate pieces and publish those parts individually to increase the number of publications.
• Expressed his old paper in new words by rewriting himself.
• Used large fragments of previously written research papers in the new form.

For pupils and students, self-plagiarism is:

• Re-submitting the results of an assignment (abstract, paper) handed in the previous semester, academic year, or at another institution.
• Using fragments of your past essays in a report or academic paper without referencing them.
• Using data from your past research in a new paper without notifying readers or the instructor.
• Publishing your modified paper in various journals.
• Translating your previous work into another language and passing off the new work as original academic material.

Copyright law protects unique authorship. When an author publishes work in an academic journal, he/she/they transfer the copyright to the publisher via contract.
The contract may guarantee that no part of the author's original work has been published elsewhere. Self-plagiarism is considered academic fraud.
Therefore, self-plagiarism violates publishers' copyright when it is a reproduction of their own documents to present them as new content. The distribution and sale rights belong to the intellectual property owner to whom the author has assigned the requests, per the contract.
The author may quote a small fragment of his work, leaving references. If he intends to mention a significant portion, doing so may be a breach of contract. It is a bad idea to cite several related paragraphs at a time. Moreover, it is even worse to substitute a citation with half of a new piece of work.
Many universities have databases of papers that students have previously submitted.
If similarities are found, the Academic Council will consider this evidence of the student's incompetence. After all, self-plagiarism:

• shows a lack of interest in creating a new, unique paper;
• means that the student does not contribute to science in a new way;
• undermines academic ethics and the public's confidence in science in general;
• violates the rights of publishers if the student's work has been previously published in a journal.

Abusing self-plagiarism can lead to expulsion or other academic sanctions.

To avoid plagiarism:

1. When writing research papers, you should not consciously use your texts. It would be best if you preferably focused on creating fundamentally new content, exploring the subject from a different perspective than the previous work.
2. Information regarding citing yourself should be indicated if there is an absolute need to include fragments of previous work in the new text.
3. Should it be necessary to resubmit your work, for example, one that has not yet been published, you should be as open as possible. It is advisable to tell the professor that you wish to submit a research paper identical to the one you were working on in the past.

If the professor cooperates, you should indicate in the introduction that the paper is based on your past research.
Sometimes it may seem that you are writing a new paper, but you are unintentionally using phrases from a past article without remembering them. So there is a risk of accidental plagiarism of your work. That's why you should use a quality plagiarism self-checker like Skandy. The checker compares your work against billions of open-source files and highlights non-unique fragments.
The service allows you to conduct a self-check for plagiarism, to detect unintentional plagiarism when you accidentally put a fragment in someone else's words. You can download and scan documents in various formats, including check pdf for plagiarism.