Maybe i wrong, but i think that is a bug!
When add category, all forms is used, show that errors but category is saved and created , when you edit that category, every is good and category work.
In this case problem is seo, but seo not have same words and system show errors for that, when seo have same words, system not show error.
Whatever is happend, show errors but create category and save.
What is point?