Archive: Avoid problems when multiple files use the same archive.

Archive: Avoid problems when multiple files use the same archive.

Matt Lundin writes:

   When I have multiple org files archive to a single file, as in

   ,----
   | (setq org-archive-location "~/2009.org_archive::")
   `----

   I find that including archive files in the agenda display (C-u v)
   results in multiple identical entries in the agenda display (in my
   case, when doing a word search).

   In fact, the number of duplicate lines exactly matches the number of
   agenda files that archive to that file (in my case 16). Is the agenda
   function perhaps searching the archive file once for each of the
   agenda files and then displaying the results as separate lines?

Indeed, when putting together the list o archive files to search,
Org did not check for duplicates.  This commit implements
uniquification of the list.