When you add a block that has instance_allow_multiple() to return false, then it's correct that when you click "+ Add a block" in the block drawer the specific block is not shown anymore. But I've found two scenarioes where more than one block still is shown.

1st Scenario, Test Instruction

• Go to a course
  • Wait a second until the whole page is loaded
• Click "+ Add a block" in the block drawer
• Click more than once on a block, that has instance_allow_multiple() set to return false
• Reload the course

This should show x amount of blocks, based on how many times you clicked. And remove the block from the options in the "+ Add a block" modal

2nd Scenario, Test Instruction

• Go to a course
• Don't wait, and immediately click on "+ Add a block" in the block drawer
• You should now have been redirected to a new page, with an URL that includes "&bui_addblock"
• From here you should be able to see all blocks, which has been added and still has instance_allow_multiple() set to return false

If you click on a block again, this should show x amount of blocks, based on how many times you clicked, again.

Impatient users could accidentally click it multiple times resulting in a lot of instances that needs to be removed or a course breaking from loading too many blocks, depending on how big the added block is.

I've tested the scenarios above on Moodle 4.0.4+, 4.0.6, 4.0.7 & 4.1