In a CV, where to display conference articles that became journal articles Some conferences select articles for publication in a journal. In my CV, I have a section "Peer-reviewed journal" and another section "Conference". Where should I put such articles?


Only in "Peer-reviewed journal": why not, but then the conference contribution will be missing
In both sections: it is factually true, but in way, artificially, increases the list. 


A similar question has been asked [1] but I understand there is a difference here: only the best conference submissions are selected for publication in a journal. It is not an automatic.

[1]  Should a conference paper in a journal be considered a journal article?

Edit Requested clarifications:


Field is mechanical engineering, where journals are more important than conference (generally).
The conferences have their own proceedings, plus a few papers are selected for an independent publication in a journal (that has somehow partnered with the conference).
Tl;dr: Put (published) conferences papers that are subsequently published as journal articles in both sections of your CV.
A journal article is a publication, as is a conference paper, regardless of whether the article was derived from the paper. Each publication is a contribution to science, so list both on your CV, they're both accomplishments. You can sort manuscripts by prestige of publication type, e.g., book, book chapter, journal article, conference paper, unpublished peer-reviewed manuscript, other works, work in progress, etc., optionally omitting some categories and providing incomplete listings for others (e.g., you might want to list only key works in progress). You can also cross-reference to avoid artificially inflating your accomplishments. For instance,

Journal articles
[1] anderstood (2020) A, Journal, 1(2), pp1-10.
[2] anderstood (2019) B with C, Journal, 5(7), pp59-76. Preliminary results appeared in [3].
Conference papers
[3] anderstood (2018) B, Conference, Published, pp20-27.

Note: I presume that journal articles derived from conference papers must add new material. This mightn't be the case for unpublished conference papers. In such cases, you could list both manuscripts and cross-reference, but it depends whether the unpublished work carries prestige, i.e., whether it is considered a (sufficiently large) accomplishment, which is field dependent.