Product Category

Hi,
I was adding a product category, unfortunately I forgot to add a slug to it, I then click to add it. When you don’t add a slug to the category, it doesn’t appear on the right hand side (where you can delete, edit etc)
Seeing it not being there, I decided to add it again, this time with a slug.
Right now I only this one category appear on the right hand side (the one with the slug).
However, when I go to the product page (where you add product info and assign category) there are two of the same category appears there.
How do I get rid of the mistaken one? As it doesn’t appear on the category page, only on the product page.