Backup of course with Quickmail block causes error

When backing up course that has latest quickmail block, receiving the following error:

error/setting_already_added

More information about this error
Stack trace:

line 333 of /backup/util/settings/base_setting.class.php: base_setting_exception thrown
line 98 of /backup/util/settings/backup_setting.class.php: call to base_setting->add_dependency()
line 11 of /blocks/quickmail/backup/moodle2/backup_quickmail_block_task.class.php: call to backup_setting->add_dependency()
line 185 of /backup/moodle2/backup_block_task.class.php: call to backup_quickmail_block_task->define_my_settings()
line 132 of /backup/util/plan/base_task.class.php: call to backup_block_task->define_settings()
line 60 of /backup/util/plan/base_plan.class.php: call to base_task->set_plan()
line 190 of /backup/moodle2/backup_plan_builder.class.php: call to base_plan->add_task()
line 95 of /backup/moodle2/backup_plan_builder.class.php: call to backup_plan_builder::build_course_plan()
line 163 of /backup/util/factories/backup_factory.class.php: call to backup_plan_builder::build_plan()
line 62 of /backup/util/plan/backup_plan.class.php: call to backup_factory::build_plan()
line 357 of /backup/controller/backup_controller.class.php: call to backup_plan->build()
line 126 of /backup/controller/backup_controller.class.php: call to backup_controller->load_plan()
line 84 of /backup/backup.php: call to backup_controller->__construct()

removing the quickmail block allows for course back up without error