In a 2012 Phd by publication, based on a book from 2010, I found a chapter where the author claims to have found some literary references (or sampling) in the literary work of someone else. However, an internet essay, published in 2005 on a forum and updated in 2006 and 2009, has exactly the same findings. But this was published by a mere 'fan' of the work, not by an academic or a scholar.
In 2010 the author claimed there was no problem because he was unaware when he wrote the book that somebody else had researched the same topic 5 years before him. In 2012, for the Phd, he simply ignored it (as the existence of the 2005 essay was several times mentioned to him after the publication of the book).
While there is perhaps no 'plagiarism' as such, shouldn't the doctor have mentioned, out of shere politeness and honesty, that somebody else had already come to the same conclusion?