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I suspect you may have read Shikigami Twilight Days (式神トワイライトデイズ), published in 2014 vol. 28, which is the one-shot predecessor to Kagamigami. In case you're interested, I found this by using this nifty Shounen Jump database and searching for the author (Iwashiro Toshiaki / 岩代俊明) in the oneshots (読み切り) section at the bottom. Kagamigami's dic.pixiv page ...


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