How do I fix dm_text_domain() error in domain mapping plugin?

1. I installed this domain mapping plugin:
http://wordpress.org/extend/plugins/wordpress-mu-domain-mapping/installation/

2. added this line to wp-config:
define( 'SUNRISE', 'on' );

(all good so far)

3. then put the domain_mapping.php file into wp-content/mu-plugins/
which completely broke the entire website - all requests returned this error:

Fatal error: Cannot redeclare dm_text_domain() (previously declared in /home/dvcreato/public_html/wp-content/mu-plugins/domain_mapping.php:29) in /home/dvcreato/public_html/wp-content/plugins/wordpress-mu-domain-mapping/domain_mapping.php on line 30

How can I fix this?

(I wanted to try this plugin before the WPMUDEV plugin... should I try that one instead?)