Piwik\Plugin\Manager::isPluginLoaded PHP Method

    public function isPluginLoaded($name)
    {
        return isset($this->loadedPlugins[$name]);
    }

 private function canPluginBeInstalled($plugin)
 {
     if (empty($plugin['isDownloadable'])) {
         return false;
     }
     $pluginName = $plugin['name'];
     $isAlreadyInstalled = $this->pluginManager->isPluginInstalled($pluginName) || $this->pluginManager->isPluginLoaded($pluginName) || $this->pluginManager->isPluginActivated($pluginName);
     return !$isAlreadyInstalled;
 }