A function to collect links in an email and group them together at the end

collect-links.el

(defun collect-links ()
  (interactive)
  (save-excursion      
    (goto-char (point-min))

    (let ((urls '()))
      ;; Find unique URLs
      (while (re-search-forward  "[a-zA-Z]+://[^\n\s-]+" nil t)
  	(add-to-list 'urls (match-string-no-properties 0)))

      ;; Add to list adds to the front. So we need to reverse
      (setq urls (reverse urls))
      ;; Replace them with a number
      (let ((counter 1))
	(dolist (url urls)
	  (goto-char (point-min))
	  (while (search-forward url nil t)
	    (replace-match
	     (concat "[" (number-to-string counter) "]")))
	  (setq counter (+ 1 counter))))

      (goto-char (point-max))
      (newline 2)

      ;; Print the numbered list
      (let ((counter 1))
	(dolist (url urls)
	  (insert (concat "[" (number-to-string counter) "] " url))
	  (incf counter)
	  (newline))))))