(defun clean-list (ll)
 "checks that it has a list, and calls clean-list1"
   (cond ((listp ll) (clean-list1 ll))
         (t (format t "not a list"))
   )
)

(defun clean-list1 (ll)
 ""
; traverses the list recursively element by element
; it builds a result list which for each duplicate element
; includes only its last occurrence in the original list
   (cond ((null ll) nil)
         (t (if (member (car ll) (cdr ll))
                (clean-list1 (cdr ll))
                (cons (car ll) (clean-list1 (cdr ll)))
            )
         )
   ) 
)