Cannot add a custom block with a block description already used by a non-reusable block

Problem/Motivation

To reproduce:

Using Drupal 8.8.x, Standard

• Enable Layout Builder and override on Basic page
• Add a node
• Override the node layout and add a Basic block inline block with the title Testing
• Save the layout.
• Add a new block on admin/structure/block/block-content and use Testing as block description.

Current behaviour:

You will now get the error: A custom block with block description Testing already exists.

However, since the block is non-reusable and a composite of that node it will not be listed in the Custom block library.

Expected behaviour:

The block should have been created.

Layout Builder / non-reusable blocks seems to ignore the UniqueField constraint on the label (which is probably correct) but it should not conflict with reusable content blocks because they are distinguished.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet