Problem/Motivation
The first version of validation was introduced in #1998658: Creating Custom Block with same name as existing block throws SQL error, you can check commit (https://git.drupalcode.org/project/drupal/-/commit/edcc75602fe1fc3b1d15049bdaec0198ce5aa43a)
At that moment the custom_block module has a custom table with "unique keys", but now it is not a case any more
(core/modules/block/custom_block/custom_block.install)
'unique keys' => array(
'revision_id' => array('revision_id'),
'uuid' => array('uuid'),
'info' => array('info'),
),
Currently custom block entity has constraint UniqueField for info field. I don't see any reason to have it unique.
We can have different content block types and of course they can have not unique info fields.
A little history
This was probably from before we had UUIDs
Back in Drupal 7 these were likely using in hook block info
But now we use the uuid, so this constraint isn't needed.
Steps to reproduce
Add a few block with the same info field value.
Proposed resolution
Get rid off UniqueField constraint.