EDIT: Seems another member was able to remove / comment the lines and the issue went away - I tested across multiple browsers, but I'm going to assume that my experiment below is caused by caching or local being weird.

Referencing conversation happening in #core-editor slack about this.

TT1 Blocks Testing
Gutenberg File Linked (template-loader.php).

The viewport meta tag is also being repeated when a wp_template CPT is loaded, not just the title tag.

<meta name="viewport" content="width=device-width, initial-scale=1">

Removed lines 94-100 in GB template-loader.php:

	// Add hooks for template canvas.
	// Add viewport meta tag.
	add_action( 'wp_head', 'gutenberg_viewport_meta_tag', 0 );

	// Render title tag with content, regardless of whether theme has title-tag support.
	remove_action( 'wp_head', '_wp_render_title_tag', 1 );    // Remove conditional title tag rendering...
	add_action( 'wp_head', 'gutenberg_render_title_tag', 1 ); // ...and make it unconditional.

Reloaded plugin, the same error occurs when saving a brand new template (saved a new one in case the old edited file did not re-render on plugin code change).

It appears like there is another file / call somewhere that is causing the duplication.

It looks like it's an issue with how the same functionality is handled in the plugin to bridge older versions of WordPress. Related part here that uses gutenberg_render_title_tag callback:

In the WordPress core it's handled with _block_template_render_title_tag :

https://github.com/WordPress/wordpress-develop/blob/b97fe776da274ed6349c795d495dcb72968e698f/src/wp-includes/block-template.php#L82-L84

The simplest approach would be to remove the wp_head action with _block_template_render_title_tag callback in the Gutenberg plugin. There might be also room for improvements in how we polyfill older versions of WordPress that don't have the functionality added in WP 5.8.

Technically, couldn't the polyfill be as simple as a WP Version check in the template loader in GB?

Technically, couldn't the polyfill be as simple as a WP Version check in the template loader in GB?

Sure, the usual approach used in the plugin is to check if the function is defined, so here if there is no _block_template_render_title_tag then you either define the same function or define a different one and register in a hook

That makes sense, at scale version tracking for different functions would become a nightmare over updates.