CMS Made Simple Forums

Hello CMSMS-community, this is my first post here. 

Usually I just lurk around, searching for previous posts about problems that occur, but on this one I found nothing:

Fatal error: Cannot redeclare smarty_cms_modifier_summarize() (previously declared in /home/httpd/vhosts/descartes.nl/httpdocs/plugins/function.summarize.php:13) in /home/httpd/vhosts/descartes.nl/httpdocs/plugins/modifier.summarize.php on line 13

Can someone tell me why this occurs and what I will have to do to fix this? It happens when opening TAGS or EVENTS, the same error.

I do not exactly know when this error occurred first, since I'm modifying a website already build by someone else.