[Plugin: NextGEN Gallery] Fatal error: Cannot redeclare class nggGallery in core.php line 9

I cannot activate the plugin because I get this error:

Fatal error: Cannot redeclare class nggGallery in /var/www/clients/client2/web6/web/wp-content/plugins/nextgen-gallery/lib/core.php on line 9

What’s up? Is the plugin really broken?

I am trying to use version 1.9.6

https://www.ads-software.com/extend/plugins/nextgen-gallery/