add_meta_boxes is called twice?

When I use add_meta_boxes hook, the hooked function appears to be called twice.

For example, writing the following code, ‘Gutenberg’ displayed top of the source cord and end of head tag.

add_action( 'add_meta_boxes', 'test_add_meta_boxes' );
function test_add_meta_boxes(){
  var_dump( 'Gutenberg' );
}

Is this a problem with this plugin, or should I do something?

Thanks.