Currently, it's possible to add multiple titles inputs, where each is a list of featured templates. The form does not allow you to save (it shows an error: "More than one property in FeaturedTemplates. Key: FeaturedTemplates"), but it shouldn't give the option.
The 'Add another element' button should not be shown if there is already one element.
Reported by @Escargot_rouge at T392896#10936457
