I have seen this error thrown up many many times while activating a wordpress plugin and the
Plugin could not be activated because it triggered a fatal error.
Fatal error: Cannot redeclare similar_posts() (previously declared in /home/pbu/public_html/blog/wp-content/plugins/xxxx/xxxx:45) in /home/pbu/public_html/blog/wp-content/plugins/x/xxxx.php on line 46
The main reason why the plugin could not be activated is because there must be either a plugin clash using same function or a wordpress theme calling the plugin function using the same name ie. double declaration.