(defun sgml-tag-again ()
  "Insert another of the tag we're in as sibling"
  (interactive )
  (let ((elt (sgml-element-name (sgml-find-element-of (point)))))
    (sgml-up-element)
    (sgml-insert-element elt)))

(define-key sgml-mode-map "\C-cn" 'sgml-tag-again)