Fatal error on setup

Evening folks.

I am running through the setup procedure for this plugin and have received the following error message.

Fatal error: Cannot redeclare dm_text_domain() (previously declared in /var/sites/w/…..cleaningservices.uk/public_html/wp-content/mu-plugins/domain-mapping.php:29) in /var/sites/w/…..cleaningservices.uk/public_html/wp-content/plugins/wordpress-mu-domain-mapping/domain_mapping.php on line 30

Can somebody help please.

Many Thanks

JD

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