Fatal Error After Chaning Themes

I was using a theme that required some plugins to be activated. I changed the theme (without disabling any plugins) and now when I go to my site (either www or admin) I get the following message:

Fatal error: Cannot redeclare get_recent_comments() (previously declared in /home/advertis/public_html/dietingdiva/wp-content/plugins/get-recent-comments.php:529) in /home/advertis/public_html/dietingdiva/wp-content/themes/didactic/functions.php on line 11

Any help would be greatly appreciated.

Thanks,
Lisette