Dispatcher: Fix subtle bug with modified items being added twice to the sort index (#9628)

Discovered by @poire-z in https://github.com/koreader/koreader/pull/9531#issuecomment-1274839234

frontend/dispatcher.lua

@@ -518,7 +518,9 @@ function Dispatcher:_addToOrder(location, settings, item)
                 end
             end
         else
-            table.insert(location[settings].settings.order, item)
+            if not util.arrayContains(location[settings].settings.order, item) then
+                table.insert(location[settings].settings.order, item)
+            end
         end
     end
 end