The original problem is that if the slideshow is embedded in the category description but not in an item published within that category it does not render correctly.
If the slideshow is embedded in the category description AND in an item published within the same category then it does render correctly in the category description (where it is required) but, as a consequence of also HAVING to publish it in an item, it then does appear twice. Once within the category description and once within the published item.