doc-140503064838

# Ruby: How to find and return a duplicate value in array?

ary = ["A", "B", "C", "B", "A"]

ary.group_by { |e| e }.select { |k, v| v.size > 1 }.map(&:first) # => ["A", "B"]

ary.sort.chunk { |e| e }.select { |e, count| count.size > 1 }.map(&:first)

ary.select { |e| ary.count(e) > 1 }.uniq # => ["A", "B"]

ary.detect{ |e| ary.count(e) > 1 } # => "A", `detect` return only the first