配列の中で重複しているものだけ抜き出す方法
arr = [1,2,3,4,5,6,7,8,9,1,2,3,4]
arr.uniq = [1,2,3,4,5,6,7,8,9]
はできる。
[1,2,3,4]だけ抽出したいときは
arr.select{|a| arr.index(a)!=arr.rindex(a)}.uniq
でできる
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