wp_insert_term() doesn't return an error, if post_tag already exists

[ocean90]

rboren > I bet its falling through down to 1649, where it returns the term without error.
Without inserting.

Because of that, on Post Tags page, the tag will be added more then once.

[ryan]

(In [14784]) If slug and name match an existing term in the same taxonomy at the same hieararchy level return error. fixes #13481