Add logic to check if thumbnail already exist

lib/tasks/thumbnail.rake

@@ -25,13 +25,14 @@ namespace :thumbnail do
         # Terminate loop if there are no more items to import
         break if items.empty?
 
-        # Increment offset, to get new items on next iteration
-        offset = offset + limit
-
         items.each do |item|
-          ThumbnailGeneratorWorker.perform_async(item.id)
+          if item.thumbnail.empty?
+            ThumbnailGeneratorWorker.perform_async(item.id)
+          end
         end
 
+        # Increment offset, to get new items on next iteration
+        offset += limit
       end
     end
   end