subfolder installation user block login and destination issue
Finally i found the issue of redirection. I have checked form.inc file from where i have found that drupal_goto function from common.inc file is called.

Now by debugging that form.inc i found that drupal_redirect_form calls drupal_goto with different options. In subfolder case (i.e. my case) drupal_goto function i have found that $path has value like '/drupal6/' default. So what i have hacked is 

$folder_name = '/drupal6/';
$length = strlen($folder_name);
$path_check = substr($path,0,$length);
$path_len = strlen($path);
$remaining_path = '';

if($path_check == $folder_name) {   
    $remaining_path = substr($path,$length,$path_len);
    echo $path = $remaining_path;
}


So now my problem of adding two folder name like 'http://localhost/drupal6//drupal6/admin/settings/site-information?destination=admin%2Fsettings%2Fsite-information/' have been solved but as you know that this is core hack and i dont like it. Any better solution???

I know that if i transfer this site from my localhost to internet than that would not create any problem without making this kind of small hacks.