When I was in college, I had an English instructor who really enjoyed the subject. His bachelor’s degree was in Marine Biology, but he happened to do some work in a lab with a man to whom the subject was a hobby – a passion. That man was named John Steinbeck.
With an inspiration as Steinbeck, my instructor’s interests turned from Marine Biology to English. He wrote a number of books including a biography of Peter Mark Roget – father of the thesaurus. One day in class my instructor brought up the subject of plagiarism. He was renown enough to be asked to review manuscripts of textbooks for English education. One such book had a poem, supposedly written by a student of the author of the book. My instructor recognized it – he had read that poem in a publication that had, as I recall, a nationwide readership of perhaps fifty people. One of those people happened to be my instructor. That was enough to stop the textbook writing career of that author dead in its tracks.
Plagiarism may be overt in the written word. In music, however, it may crop up subliminally. A songwriter may get a chord progression in his head and he may go with it, not realizing it had been heard before.
Often times it is not a conscious act on the part of a songwriter to rip off from another songwriter, but it can happen despite one’s best intentions.