You can reproduce this even with Twenty Nineteen, by adding <?php $blocks = gutenberg_parse_blocks( $post->post_content ); ?> to header.php.

Replacing gutenberg_parse_blocks() calls with parse_blocks() seems to work around the issue (for me at least), and the issue here, if any, is apparently just one of confusion: renaming the function broke backwards compatibility, but since gutenberg_parse_blocks() is still defined, instead of a ”Call to undefined function” you get this attempt to redefine WP_Block_Parser (via inclusion of parser.php), resulting in the ”name is already in use”.