Upgrading from 5.x to 6.x

This looks to happen because:

In dxpr_theme_post_update_n1_migrate_colors we

  // Load callbacks.
  require_once $theme_handler
    ->getTheme('dxpr_theme')
    ->getPath() . '/dxpr_theme_callbacks.inc';

Which gives us the function 'dxpr_theme_css_cache_build'

Then we loop over all themes and if it is dxpr_theme or a first generation child subtheme - i.e. this update doesn't work for multiple levels of subtheme.

We do stuff then call dxpr_theme_css_cache_build

In dxpr_theme_css_cache_build we

Use a hardcoded reference to the dxpr_theme to load all the files in features that are suffixed css.inc

foreach (\Drupal::service('file_system')->scanDirectory(\Drupal::service('extension.list.theme')->getPath('dxpr_theme') . '/features', '/css.inc/i') as $file)

This loop runs multiple times if we have a subtheme/s that match above.

We are using 'require_once $file->uri;' so we should only require this once. ????? SO why do we get a redeclared error!

used function_exists to fix the issue