It's possible to rename a custom pattern categories using existing category names

[jameskoster]

Follow-up to #55035.

To reproduce:

1. Create a custom pattern category (add new pattern, type a non-existing category name into Categories input)
2. Rename category (via ellipsis menu in top right)
3. Choose an existing category name
4. See duplicate categories in the Pattern management sidebar:

Potential solutions

1. Validate and do not allow duplicate names (this would be consistent with the post category experience)
2. Allow duplication, but append the name with -1, (Copy), or similar.

I'd lean towards the first option, but keen to hear other thoughts and ideas.