Duplicate slug causes multiple posts to be displayed in single post template

[SergeyBiryukov]

This is very similar to #22902, but affects hierarchical post types. See ​#meta523.

Steps to reproduce:

1. Create a hierarchical CPT, e.g. "handbook".
2. Add a top-level post called "Getting Started".
3. Add another top-level post called "Contributing to the WordPress Codex".
4. Add a third post called "Getting Started" and select the previous post as a parent.
5. You should get this structure as a result:

   /handbook/getting-started/
   /handbook/contributing-to-the-wordpress-codex/
   /handbook/contributing-to-the-wordpress-codex/getting-started/
   

6. Now go to /handbook/getting-started/. Instead of just a top-level "Getting Started" post, you'll see two posts with the same title displayed in single.php template.

Has been this way since [13774].

WP_Query has an assumption​ that a top-level post of a hierarchical CPT can use name query var. However, we don't check for duplicate slugs in this case. Looks like hierarchical post types should always use pagename.

[SergeyBiryukov]

Fixed in [28803].

[SergeyBiryukov]

In 28809:

Use assertCount() for consistency with other tests.

see #28611.