We have created a plugin that creates multiple sites. Since New Blog Templates is activated when each of the sites is created, the "set_blog_defaults" function is called and the "copier.php" file is included each time. Due to multiple sites being created, the following error occurs indicating that the class within "copier.php" is being redeclared each time:
Fatal error: Cannot redeclare class NBT_Template_copier in /public_html/wp-content/plugins/blogtemplates/blogtemplatesfiles/copier.php on line 5
As simple solution to this is to change "include('copier.php')" on line 344 of ./blogtemplates/blog_templates.php into require_once('copier.php'). Also, wrapping the "NBT_Template_copier" class in a "if( !class_exists('NBT_Template_copier') )" would also be advised.