Validation for UseOnce blocks when switching from code to visual

[hedgefield]

Issue Overview

I can add multiple instances of a UseOnce block to a post by copy-pasting in the code editor. They don't get removed when switching back to the visual editor (or when publishing the post).

Steps to Reproduce (for bugs)

1. Start a post
2. Switch to the code editor
3. Add a Read More block via the inserter (after this it won't allow me to add a 2nd one this way, so that's good)
4. Copy-paste the block tag several times
5. Switch to the visual editor and see multiple Read More blocks with no warnings or notices

Gutenberg 2.1 + WP 4.9.4

Expected Behavior

To run validation when switching back from the code editor to remove the extra blocks, or show some sort of notice.

Pinging @mcsf as we discussed this during the core-editor chat.

[mcsf]

@hedgefield, have a look at #5031, let me know what you think.