(defun find-elt (elt list)
  (cond ((null list) nil)
        ((= (car list) elt) t)
        (t (find-elt elt (cdr list)))))

(defun duplicatep (list)
  (cond ((null list) nil)
        ((find-elt (car list) (cdr list)) t)
        (t (duplicatep (cdr list)))))



LIST-NAMED-CLASS> (duplicatep '(1 2 3 4 5 6 7 8 9 10))
NIL
LIST-NAMED-CLASS> (duplicatep '(1 2 3 4 5 6 7 8 9 10 1))
T
LIST-NAMED-CLASS> (duplicatep '(1 2 3 4 5 6 7 8 9 10 8))
T



LIST-NAMED-CLASS> (trace duplicatep find-elt)

LIST-NAMED-CLASS> (duplicatep '(1 2 3 4))
  0: (LIST-NAMED-CLASS::DUPLICATEP (1 2 3 4))
    1: (LIST-NAMED-CLASS::FIND-ELT 1 (2 3 4))
      2: (LIST-NAMED-CLASS::FIND-ELT 1 (3 4))
        3: (LIST-NAMED-CLASS::FIND-ELT 1 (4))
          4: (LIST-NAMED-CLASS::FIND-ELT 1 NIL)
          4: FIND-ELT returned NIL
        3: FIND-ELT returned NIL
      2: FIND-ELT returned NIL
    1: FIND-ELT returned NIL
    1: (LIST-NAMED-CLASS::DUPLICATEP (2 3 4))
      2: (LIST-NAMED-CLASS::FIND-ELT 2 (3 4))
        3: (LIST-NAMED-CLASS::FIND-ELT 2 (4))
          4: (LIST-NAMED-CLASS::FIND-ELT 2 NIL)
          4: FIND-ELT returned NIL
        3: FIND-ELT returned NIL
      2: FIND-ELT returned NIL
      2: (LIST-NAMED-CLASS::DUPLICATEP (3 4))
        3: (LIST-NAMED-CLASS::FIND-ELT 3 (4))
          4: (LIST-NAMED-CLASS::FIND-ELT 3 NIL)
          4: FIND-ELT returned NIL
        3: FIND-ELT returned NIL
        3: (LIST-NAMED-CLASS::DUPLICATEP (4))
          4: (LIST-NAMED-CLASS::FIND-ELT 4 NIL)
          4: FIND-ELT returned NIL
          4: (LIST-NAMED-CLASS::DUPLICATEP NIL)
          4: DUPLICATEP returned NIL
        3: DUPLICATEP returned NIL
      2: DUPLICATEP returned NIL
    1: DUPLICATEP returned NIL
  0: DUPLICATEP returned NIL
NIL

LIST-NAMED-CLASS> (duplicatep '(1 2 3 4 2))
  0: (LIST-NAMED-CLASS::DUPLICATEP (1 2 3 4 2))
    1: (LIST-NAMED-CLASS::FIND-ELT 1 (2 3 4 2))
      2: (LIST-NAMED-CLASS::FIND-ELT 1 (3 4 2))
        3: (LIST-NAMED-CLASS::FIND-ELT 1 (4 2))
          4: (LIST-NAMED-CLASS::FIND-ELT 1 (2))
            5: (LIST-NAMED-CLASS::FIND-ELT 1 NIL)
            5: FIND-ELT returned NIL
          4: FIND-ELT returned NIL
        3: FIND-ELT returned NIL
      2: FIND-ELT returned NIL
    1: FIND-ELT returned NIL
    1: (LIST-NAMED-CLASS::DUPLICATEP (2 3 4 2))
      2: (LIST-NAMED-CLASS::FIND-ELT 2 (3 4 2))
        3: (LIST-NAMED-CLASS::FIND-ELT 2 (4 2))
          4: (LIST-NAMED-CLASS::FIND-ELT 2 (2))
          4: FIND-ELT returned T
        3: FIND-ELT returned T
      2: FIND-ELT returned T
    1: DUPLICATEP returned T
  0: DUPLICATEP returned T
T