I found out that a collegue published about 5 years ago a paper in the proceedings of a conference about education in engineering (proceedings has ISBN, but publication has no DOI, in case that matters for this question). Then reworded the whole paper but kept literally the same references, same research questions, same arguments all over the paper and published in another conference (IEEE, this time with a DOI). So I imagine he purposely reworded and made up two papers with "the same" content, though he obviously rewords paragraph by paragraph (changes sentence order, synonyms, etc.). I thought this practice was not allowed but does anyone know the legality of publishing the same paper (though reworded) in two conferences?
I don't know of any legal regimen in which self plagiarism is an issue. It is scientific misconduct, but not illegal, to use your own words and ideas without citing them. This is a case in which the legal and the ethical aren't the same.
It isn't "stealing" ideas when you self plagiarize. It is just sloppy scholarship that makes it harder for readers and other scholars to find the complete context of the ideas expressed.
Of course, if you have given up copyright to some words, then using those words again may violate the copyright that you once held, but no longer do. A publisher that holds the copyright might make a legal claim against you, but it isn't because of self-plagiarism, but copyright violation.
I don't think there is anything illegal. However, it does sound like an academic misconduct, namely case of Duplicate Publication.
Duplicate publication, multiple publication, or redundant publication refers to publishing the same intellectual material more than once, by the author or publisher. It does not refer to the unauthorized republication by someone else, which constitutes plagiarism, copyright violation, or both.
Scientific ideas presented multiple times, even reworded, are not useful for community, and might distort impression of your output. At best, you waste your own time, at worst you are wasting time of others by presenting old research as novel.
On the other hand, I can imagine someone writing a "review" of their own paper, but then they still have to cite the original.