lispdoc - results for delete-duplicates

(delete-duplicates sequence &rest args &key (test #'eql) (test-not nil)
 (start 0) (end nil) from-end (key nil))
Function: The elements of SEQUENCE are examined, and if any two match, one is discarded. The resulting sequence, which may be formed by destroying the given sequence, is returned. The :TEST-NOT argument is deprecated.
Example:
(defun free-variables (form)
  (delete-duplicates (free-vars form nil) :test
                     #'eq))
Mentioned in:
CLtL2 - 14.3. Modifying Sequences
HyperSpec - Function REMOVE-DUPLICATES, DELETE-DUPLICATES
PCL - destructive operations
PCL - the heart of a spam filter
PCL - what frames do you actually need