Hi,
You'll probably need to increase the allowed memory usage on your web server. Check with your web hosting company if that's possible, or just try to add the following line to your .htaccess file:
php_value memory_limit 96M
As regards duplicate plugins, I'd recommend uninstalling JoomSEF once more, check if the plugins remain and if needed, uninstall them too. After that install JoomSEF back.