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http://jewelrybox.wpblog.jp/2017/10/01/server-already-running%e3%81%a7%e3%82%b5%e3%83%bc%e3%83%90%e3%81%8c%e8%b5%b7%e5%8b%95%e3%81%a7%e3%81%8d%e3%81%aa%e3%81%84%e3%81%a8%e3%81%8d%e3%81%ae%e5%af%be%e5%87%a6%e6%b3%95-rails5/
A server is already runningでサーバが起動できないときの対処法 Rails5