Collapsible Block: Generated Anchor ID's are not always unique

[acketon]

The current method of generating unique ID's is not robust enough and can lead to duplicate ID's. This is a known issue in Gutenberg and there are some improvements under consideration to allow non-duplicatable attributes. But until then, we still have an issue when a block is duplicated the "unique" ID generated from the clientId is also duplicated and then this becomes an accessibility issue and an issue for any JS on the frontend interacting with the block.

See:

• Duplicate Block - Unique ID not updating WordPress/gutenberg#17536
• Block API: Allow for internal, non-duplicable block attributes  WordPress/gutenberg#29693
• https://github.com/WordPress/gutenberg/blob/trunk/packages/block-library/src/heading/autogenerate-anchors.js
• Explore easier ways to declare and use html ids within blocks WordPress/gutenberg#17246 (comment)