Odd behaviour of tags and category names

[mrmist]

There seems to be some inconsitency around slug names for tags and categories.

If I have a tag with a slug of, say "mytag" -

I go to categories, attempt to edit an existing category and change its slug to "mytag" I'll get an error "category not updated".

I go to categories, attempt to create a new category with slug "mytag" it will create it, and the two entities will then become oddly merged together, changing the display name of one affects the other.

I do not know what - if any - of this is the expected behaviour, but it can't all be right.

I think this may be because of is_term() oddness, Could be related to #6593

Punting along with #6593

Closing as dup of #6593, #5034 and #6211