src/Pure/System/build.ML

           end;
 
       val document_variants =
         map Present.read_variant (space_explode ":" (Options.string options "document_variants"));
       val _ =
+        (case duplicates (op =) (map fst document_variants) of
+          [] => ()
+        | dups => error ("Duplicate document variants: " ^ commas_quote dups));
+      val _ =
         Session.init do_output false
           (Options.bool options "browser_info") browser_info
           (Options.string options "document")
           (Options.bool options "document_graph")
+          (Options.string options "document_output")
           document_variants
           parent_name name
           (Options.string options "document_dump",
             Present.dump_mode (Options.string options "document_dump_mode"))
           "" verbose;