After installing this plugin, I get the following message:

Plugin could not be activated because it triggered a fatal error.


Fatal error: Cannot redeclare topsy_init() (previously declared in httpdocs/wp-content/plugins/topsy/topsy.php:23) in httpdocs/wp-content/plugins/topsy/topsy.php on line 24

lines 22-24 of topsy.php:

function topsy_init() {
load_plugin_textdomain('topsy', PLUGINDIR . '/' . dirname(plugin_basename(__FILE__)) );
}


What could be causing the error?

Sounds like there is a problem with the plugin. You should contact the plugin author