Fatal Error

I’ve had this plugin running for nearly a year with no trouble. I recently had to deactivate it for unrelated testing purposes, and now I get this error when I try to reactivate the plugin:

Plugin could not be activated because it triggered a fatal error.
Fatal error: Cannot redeclare dm_text_domain() (previously declared in /www/wp-content/mu-plugins/domain_mapping.php:29) in /www/wp-content/plugins/wordpress-mu-domain-mapping/domain_mapping.php on line 30

Any thoughts?

https://ww.wp.xz.cn/plugins/wordpress-mu-domain-mapping/