;;; Public API entry point.
;;; This is just a convenient wrapper
(defun duplicate-entries (l)
  (duplicate-entries-internal l l))

;;; For all each item in items, check and see if there are more than
;;; once instance of item in l.
(defun duplicate-entries-internal (items l)
  ;; n - the # of instances of an item
  ;; item - the current item
  (let ((n 0) (item (car items)))

    ;; Define a function to count the # of instance of a.
    ;;
    ;; This is used as a predicate for member function to count the
    ;; number of items. We *only* return t if there n > 1. This makes
    ;; member function return a non-nil list iff there are more than one
    ;; instance of a.
    (defun more-than-one(a b)
      (if (equal a b)
        (if (> (incf n) 1)
          t)))

    ;; Make sure that we have something to count.
    (if (> (list-length items) 0)
      ;; Are there more than one instance of item in l?
      (if (equal (member item l :test #'more-than-one) nil)
        ;; If not, check other items.
        (duplicate-entries-internal (rest items) l)
        ;; Otherwise, we found an item with more than one instance.
        t))))

;; Test and print the results
(defun test (in) (format t "~3S = ~S~%" (duplicate-entries in) in))
;; A sample test set
(setf testset '((a b a c d) (a b (a) c d) ((a b) b c (a b))))
;; Run duplicate-entries over all test samples
(mapcar #'test testset)