org-archive.el: Fix org-add-archive-files to correctly de-duplicate

* org-archive.el (org-add-archive-files): Use `seq-uniq' with TESTFN
`file-equal-p' to de-duplicate the list of gathered files.

Previously, `org-uniquify' was used.  This de-duplicates the
file-names, but not necessarily the files.  The problem occurs if the
list of file-names includes distinct file-names that reference the
same file (symbolic links, for example).

TINYCHANGE

lisp/org-archive.el

@@ -157,7 +157,7 @@ archive location, but not yet deleted from the original file.")
   "Splice the archive FILES into the list of files.
 This implies visiting all these files and finding out what the
 archive file is."
-  (org-uniquify
+  (seq-uniq
    (apply
     'append
     (mapcar
@@ -166,7 +166,9 @@ archive file is."
 	   nil
 	 (with-current-buffer (org-get-agenda-file-buffer f)
 	   (cons f (org-all-archive-files)))))
-     files))))
+     files))
+   #'file-equal-p
+   ))
 
 (defun org-all-archive-files ()
   "List of all archive files used in the current buffer."