On current master, to reproduce:

• in the sidebar, open Categories & Tags
• click on "Add new category" at the end of the checkboxes list
• enter a new category name, for example "qwerty"
• click the button "Add Category"
• enter again the same name "qwerty"
• click the button "Add Category"
• the UI doesn't show any feedback but in the console you will see:

POST http://src.wordpress-develop.test/wp-json/wp/v2/categories 400 (Bad Request)
GET http://src.wordpress-develop.test/wp-json/wp/v2/categories/[object%20Object] 404 (Not Found)

Expected:

• either no visual feedback (the classic editor doesn't show any error, it just ignores the whole thing)
• or a proper error message displayed in the UI
• in both cases the JS errors should be fixed