From: "Mateusz Czerwiński" <mtczerwinski@...>
Date: 2013-01-13T05:13:21+09:00
Subject: Re: uniq vs uniq!

Because
  uniq return new array
  uniq! return new array or nil if no changes are made.
More info and examples you can get here
http://www.ruby-doc.org/core-1.9.3/Array.html#method-i-uniq
Regards,
Mateusz