When you try to restore the backup of a site collection multiple times to the same content database using stsadm, you may get the error:

"No content databases are available for restoring this site collection. Create a new content database and then try the restore operation again."

In the Microsoft Site its has been mentioned that to restore the same site collection multiple times, new content databases have to be created. This is because, the globally-unique identifiers (GUID) for lists are preserved in the backup file and reused during restore, but the content database requires list GUIDs to be unique. Therefore, you cannot restore a site collection twice to the same content database, and must instead use a different content database.

There are two workarounds for this issue:

1. Make the site you want to restore multiple times, as a template and apply the template to new sites, so that all the data in the template is also carried over to the new site.

2. If you don't want to make the site as a template, then follow the steps to restore into the same content database.
   (i). Create the site (destination site, into which you want to restore the source site collection) using the Central Administration Page -> Create Top Level Website. Do not apply any template to this site.
   (ii). Backup the source site as a FWP (Frontpage Web Package) using the SMigrate Tool.
   (iii). Now restore this backedup site to the new site (created without applying template) using SMigrate Tool. The site collection is restored on to the same content database.

   One thing to be noted here is that the Site Settings are not restored and you have to add the users to the site manaully (whom you want to access the site), whereas with stsadm restoration, the users are also restored. Still, instead of creating new content database each time you want to restore the same site, adding users sounds much better. Doesn't it?

//Akila.