Steps to reproduce

1. On a WordPress site that has access to wp-cli and some menu items to test with, open up the CLI for that site.
2. Run menu list to get a list of menus.
3. Run menu item list <menu> --fields=db_id,title,menu_item_parent showing the optional menu_item_parent CLI output field. This will list out a particular menu's items.
4. Run menu item update <db-id> --parent-id=<db-id> setting a particular menu item's parent-id to itself.
5. Open that menu up for viewing in the Customizer.

What I expected

• For the menu in question to load in the Customizer even with the duplicate menu/parent id.

What happened instead

• Customizer will freeze browser upon navigating to the menu in question.
• You are able to view the menu in WP Admin without freezing the browser.

Browser / OS version

Chrome Version 75.0.3770.142 (Official Build) (64-bit)

Screenshot / Video

Video: https://cld.wthms.co/RAxjnq

Context / Source

p9F6qB-3HU-p2
#user-report