Footnotes: Allow validated XHTML with multiple references to one note

Org-mode allows multiple references to the same footnote.  However,
the HTML exporter cannot use the same name for all these references,
because names have to be unique.  This patch appends numbers to
footnote reference names, to make sure they remain unique.

The dark side of this patch means that from the footnote definition,
there is only a single link, to the first reference of the note.
However, in a browser you can always press BACK to go back after
jumping to a footnote definition.

lisp/org-exp.el

@@ -3181,7 +3181,7 @@ PUB-DIR is set, use this as the publishing directory."
 	 ind item-type starter didclose
 	 rpl path attr desc descp desc1 desc2 link
 	 snumber fnc item-tag
-	 footnotes
+	 footnotes footref-seen
 	 )
 
     (let ((inhibit-read-only t))
@@ -3583,12 +3583,18 @@ lang=\"%s\" xml:lang=\"%s\">
 	    (while (string-match "\\([^* \t].*?\\)\\[\\([0-9]+\\)\\]" line start)
 	      (if (get-text-property (match-beginning 2) 'org-protected line)
 		  (setq start (match-end 2))
-		(let ((n (match-string 2 line)))
+		(let ((n (match-string 2 line)) extra a)
+		  (if (setq a (assoc n footref-seen))
+		      (progn
+			(setcdr a (1+ (cdr a)))
+			(setq extra (format ".%d" (cdr a))))
+		    (setq extra "")
+		    (push (cons n 1) footref-seen))
 		  (setq line
 			(replace-match
 			 (format
-			  "%s<sup><a class=\"footref\" name=\"fnr.%s\" href=\"#fn.%s\">%s</a></sup>"
-			  (match-string 1 line) n n n)
+			  "%s<sup><a class=\"footref\" name=\"fnr.%s%s\" href=\"#fn.%s\">%s</a></sup>"
+			  (match-string 1 line) n extra n n)
 			 t t line))))))
 
 	  (cond