I don't know if it's 100% correct, but based off the precedent set by doujinshi galleries like Yassy's "SanaSana Snacks" (https://e-hentai.org/g/1306010/a95712637f/) I'll go ahead and tag "selfcest" for the scenario in chapter 9. To be clear, what's happening is that this amateur cosplayer happens to pick the same costume as this mean-looking pro, and then totally upstages her, and they get up to wacky hijinks from there. EDIT: Neither character actually touches eachother. Maybe it's different to what turns people on about actual selfcest on this site though. In some points of view it could be closer to twins if anything, but that tag's a slave of "incest", seeing as real twins should naturally be siblings.