title: Why does union not return a unique list?
body: <p>Is there a logical reason why the following statement from the Hyperspec is the way it is?
  "If there is a duplication between list-1 and list-2, only one of the duplicate instances will be in the result. If either list-1 or list-2 has duplicate entries within it, the redundant entries might or might not appear in the result."</p>
  
  <p>Until I read this I was assuming that union should return a unique list and frustrated why my code didn't do so. It also seems odd to remove duplicates between lists but not within. Why even specify this?</p>
  
  <p>It seems like one should be able to assume union will produce a unique list of the set's elements or am I missing something?</p>
  
  <p>For the full page in Hyperspec see <a href="http://clhs.lisp.se/Body/f_unionc.htm" rel="nofollow">http://clhs.lisp.se/Body/f_unionc.htm</a></p>