Describe the bug
After my Wordpress installation updated to WordPress 5.3-alpha-46118 overnight, I found that my blog wasn't working anymore. Managed to make my way to a recovery mode where I was informed that Gutenberg (6.4.0) had not activated properly. Deactivated Gutenberg, and when I tried to activate it again, I got the following error

Fatal error: Cannot declare class WP_Block_Styles_Registry, because the name is already in use in /home/eugenemah/v16.imablog.net/wp-content/plugins/gutenberg/lib/class-wp-block-styles-registry.php on line 14

Same thing when I deleted Gutenberg, reinstalled, and tried to reactivate

Had been working fine under the previous alpha version (don't recall what number that was)

One of my other blogs still running Wordpress 5.2.3 is still operating normally.

To reproduce
Steps to reproduce the behavior:

1. Go to 'Plugins'
2. Click on 'Activate' under the Gutenberg plugin
3. See error

Expected behavior
Gutenberg plugin is activated

Desktop (please complete the following information):

• OS: LInux 4.14.117-grsec-grsec+ (Dreamhost)
• Browser: Chrome-dev
• Version: 78.0.3904.9