I’m interested in this because I have forums where each category is used by a different group. In the main, categories will have the same sub-categories so this issue will arise when I use the Category Icon theme component. But it seems likely that all the duplicated subcategories will have the same icon and other settings so I probably won’t notice the underlying problem you describe.
I don’t like the theme component assuming unique slugs when they are not. But I can’t think why I would have different icon requirements for subcategories with matching slugs.