Duplicate the elements of a list

	;; P14 (*) Duplicate the elements of a list.
	;; Example:
	;; * (dupli '(a b c c d))
	;; (A A B B C C C C D D)
	(define (dupli lst)
	(let loop ((lst lst) (acc '()))
	(if (null? lst)
	(reverse acc)
	(let ((head (car lst)))
	(loop (cdr lst)
	(append `(,head ,head) acc))))))

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