(define (f ns)
  (define (aux m1 m2 ns)
    (if (null? ns)
        m2
        (let ((n (car ns)) (ns (cdr ns)))
          (cond
           ((not m1) (aux n #f ns))
           ((or (and (not m2) (< m1 n)) (< m1 n m2)) (aux m1 n ns))
           ((or (and (not m2) (< n m1)) (< n m1 m2)) (aux n m1 ns))
           (else (aux m1 m2 ns))))))
  (aux #f #f ns))
(print (f '(4 5 1 7 1 2 8 9 2 7)))
(print (f '(1 1 1)))
(print (f '(1)))
(print (f '()))
 

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