I tried to search in the previous Q/As, but I do not think my case falls in any of the previous entries.
While looking for a certain topic in my field, I found two papers. The first paper was authored by authors A, B, C, and D and published in some conference proceedings. The second paper was published in a journal in a year after by authors A, E, D, and C. That is, the second paper lost one author and gained a new author.
The two papers report the same research hypotheses, the same data collected, the same data analysis, the same results. That is, the "numbers" in the papers are the same (even the table contents).
The texts differ in a subtle way. The first paper is entitled "Studying topic T in the domain D". The second paper is entitled "Using Measurement Instrument M to measure T in the domain D". The second paper somehow avoids copy/pasting of the paragraphs from the first paper-they rewrote the sentences. It is a little bit longer than the first one. The authors actually spent some more words talking about measurement instrument M, which is taken from another discipline and it takes 3 minutes of Google Scholar searching for learning more about it. There is even a Wikipedia page about the instrument.
I think that this is a case of self-plagiarism. The second paper does not add anything useful to the first one. Additionally, the second paper does not cite the first one. This ringed a bell.
Please note that in my field, conferences are archived and are often considered more important than journals. Regardless of which venues are more important, both papers are formally considered publications.
Now, am I morally obliged to contact the editor of the journal where the second paper was published? This is not precisely a double publication, nor are the authors damaging me nor anybody else. Still, I feel they did something wrong. Should I instead live and let die?