Attachment slugs are not unique

[duck_]

From wp_unique_post_slug() and #9726:

Attachment slugs must be unique across all types.

It's true that attachment slugs are checked for uniqueness against all other object types, however, when inserting an object of another type any uniqueness checks do not include attachments. This means that #9539 isn't actually fixed if you attempt the steps in the order described (insert attachment first followed by conflicting page).

Fix would be to add 'attachment' to the post_type IN clause for hierarchical types. See attachments-hierarchically-unique.diff.

Still puzzling out why/if attachments need to be unique against non-hierarchical types since there shouldn't be collisions as attachment URLs are subpage style. However, if attachment slugs do need to be globally unique then see attachments-globally-unique.diff (not tested so much).

Related: #9726, #9539, #6437, [11467]