even i changed mysql name and user but still this coming

Zohaib, this error means you have two users with the same username. You need to delete one.
If you have an access to the admin panel, go to the browse users page and delete this user Pakistan786, otherwise, go to your phpmyadmin/ow_base_user table find and delete that user from there.

where i can fix it?

Zohaib, as I said you need to go to your phpmyadmin/ow_base_user table find and delete that user from it.
Please, specify what step of this manual is causing your difficulties?

Taissa, now this is coming.

Zohaib, please, specify did you delete the duplicate username ?
There can be a lot of reasons why this happens: http://www.oxwall.org/...%20has%20gone%20away

So the reason that your mysql server is being down may be that you're using wrong access details for the database, please check ow_includes/config.php file.
Or you may have, for example, a low memory limit on your server so please contact your hosting provider about that issue.