The slug is modified against user will when inserting a new term

[Chouby]

Create a post tag:
*name: English
*slug: en
The tag is normally created.
Now create a category:
*name: English
*slug: english
The category is created with the slug "en" (no new term is created)
Now create a category:
*name: English
*slug: english-2
The category is correctly created with the slug "english-2" (a new term is created)

Related but somewhat different from: #5809

Duplicate of #21950.