What problem does this address?

Currently we have a definition in the block.json to say supports: { "multiple": false }. This in theory should prevent a block to appear or to be added more than once. However this doesn't actually do that. Instead it just displays a message stating 'This block can only be used once.' but in fact it will still appear in the editor and in the frontend.

What is your proposed solution?

I think it would be more intuitive if the block is hidden from the inserter once there is already a block present. In that way, you can't even add another block to the page.