(defun delete-region-before-insert (orig-fun &rest args) (cl-letf (((symbol-function 'insert) (lambda (arg) (if (region-active-p) (delete-region (region-beginning) (region-end))) (insert arg)))) (apply orig-fun args)))
(advice-add 'helm-copy-to-buffer :around #'delete-region-before-insert)

(defun helm-copy-to-buffer ()
  "Copy selection or marked candidates to `helm-current-buffer'.
Note that the real values of candidates are copied and not the
display values."
  (interactive)
  (with-helm-alive-p
    (helm-run-after-exit
     (lambda (cands)
       (with-helm-current-buffer
         (if (region-active-p) (delete-region (region-beginning) (region-end)))  ; added to delete region if any.
         (insert (mapconcat (lambda (c)
                              (format "%s" c))
                            cands "n"))))
     (helm-marked-candidates))))