We often need to find the dulicate values in array.

Like the follows:

["A", "B", "C", "B", "A"]    # => "A" or "B"
["A", "B", "C"]              # => nil

How to do it?

arr = ["A", "B", "C", "B", "A"]
arr.detect{ |e| arr.count(e) > 1 }

FYI: