The so-called copyright violation was that Max Planck had published the same article in 2 journals, which was not unusual at that time, because different journals had different readerships, so publishing in more journals was necessary if you wanted to reach more people.<p>So supposedly he plagiarized himself.<p>The second retracted article was even less justifiable, because the modern editors or their automated system had believed that 2 articles were the same, but they were not, they only happened to have the same title.
While commonly taught in academic settings, I disagree with the notion that it's possible to self plagiarize. It's your own words and not stealing from somebody else.
Also that can't be the whole story because Planck died in 1947 and in Germany (then and now) Copyright ends 70 years after the death of the author.