Seriously though, I have seen a case where a Wikipedian slapped a copyvio tag on something because it shared some phrases with a webpage. The author complained that he had spent hours reading multiple sources, and rewriting the information therein in his own words. That is unequivocally acceptable under copyright law, and the tag was soon removed.
I find it hard to believe that one could have accurately found the original source of the text without the copy being copyright infringement, especially if the author had actually used "his own words". Of course, the specific details are important, but to say that it is "unequivocally acceptable" is grossly inaccurate.
I've also seen situations where an author has claimed to have spend hours reading multiple sources and rewriting the information using his own words, but then after looking at the page history it becomes obvious that he just copied the words and then changed some of them.
There's no need to be paranoid. We should be careful not to accuse people of plagiarism who are merely paraphrasing or rewriting.
Paraphrasing someone without attribution *is* plagiarism. It isn't copyright infringement, if you've done it properly, but it's still plagiarism. Even if you are only paraphrasing someone, you're supposed to cite your source.
I'm not sure if it's feasible to identify the original source of a work by checking phrases in an automated manner, but if we do in fact find the original source of a work, and that source isn't already identified, we absolutely need to do something. Whether that means identify the source or simply delete as a copyright infringement depends on both the similarity of the two works and the license status of the original.
Anthony
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