From: "Peña, Botp" <botp@...>
Date: 2007-08-21T17:59:56+09:00
Subject: Re: Detecting duplicates in an array, anything in the standa

From: Robert Klemme [mailto:shortcutter@googlemail.com] 
# irb(main):002:0> a.uniq.select{|e| (a-[e]).size < a.size - 1}

compare also,

irb(main):056:0> b=a.dup
=> [1, 1, 2, 2, 2, 4, 3]
irb(main):057:0> b.uniq.select{|e| (b.reject!{|f| f == e}).size > 1}
=> [1, 2]