gistfile1.rb

ruby-1.9.3-p0 :011 > base = {:a => "a", :b => "b"}
 => {:a=>"a", :b=>"b"} 
ruby-1.9.3-p0 :012 > other = base
 => {:a=>"a", :b=>"b"} 
ruby-1.9.3-p0 :013 > other[:c] = "c"
 => "c" 
ruby-1.9.3-p0 :014 > base
 => {:a=>"a", :b=>"b", :c=>"c"} 
ruby-1.9.3-p0 :015 > 

#--------------------------

ruby-1.9.3-p0 :015 > other =  base.dup
 => {:a=>"a", :b=>"b", :c=>"c"} 
ruby-1.9.3-p0 :016 > other[:d] = "d"
 => "d" 
ruby-1.9.3-p0 :017 > base
 => {:a=>"a", :b=>"b", :c=>"c"} 
ruby-1.9.3-p0 :018 >