Erik Naggum wrote:

I prefer to use KEY and TEST keyword arguments that default to #'IDENTITY
and #'EQL.

(defun duplic8 (element list &key (test #'eql) (key #'identity))
(loop for x in list
collect x
when (funcall test (funcall key x) element) collect x))

Scheme

(append-map
(lambda (x) (if (equal? x 'M) (list x x x) (list x)))
'(i M j l))

===>
(i M M M j l)

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