Using Joomla 3.8.2, PHP 7.0.98, Kunena 5.0.12.
Background info: I changed the name of category #1 and its alias to match a new category name #2. (Purpose was to make it into a new subcategory.) Then I wanted to create a new category #3 using the old category #1 name and alias. However, Kunena would not save the new category #3 because it said the alias was already in use. However, that alias (old category #1) was not in use because I had changed it when I changed the name and alias of category #1. I searched Kunena categories, Joomla content categories, and mySQL, and could not find the old alias for category #1 anywhere.
Solution: Finally I looked at Category #2, again. (It used to be category #1). Underneath the category #2's new alias, were some check boxes. Sure enough, one of the check boxes still contained the old category #1 alias directly underneath the new category #2 alias. So, I unchecked the check box, and the old category #1 alias disappeared. Then I could reused the old category #1 alias for a new category #3 just as I had been trying to do.
