Well you can, but first you have to remove the site collection you backed up. This code will do it
Backup-Backup-SPSite –Identity http://sharepoint.abc.com/sites/testsite -Path “c:\site collection backups\testsite.bak”
Remove-SPSite –Identity http://sharepoint.abc.com/sites/testsite
Restore-SPSite –Identity http://sharepoint.abc.com/sites/testsite2 -Path “c:\site collection backups\testsite.bak”http://sharepoint.stackexchange.com/questions/31048/restore-spsite-reports-no-content-databases-are-available-for-this-operation
Now the reason for this is a bit tricky. Remember that everything in the same site collection sits in the same database instance. SharePoint puts a lot of GUIDs in to a database. So if you try to take a backup of a site collection and restore it to another site collection in the same app you get the error:
Restore-SPSite : No content databases are available for this operationEven if you delete the site collection from the Web Application it will not work. You need to run the remove-SPSite command to remove the GUIDs from the site collection.
This is covered in Microsoft support post
Error message when you try to restore a Windows SharePoint Services 3.0 site collection by using a different name: "No content databases are available for this operation" http://support.microsoft.com/kb/926321
This post is for SharePoint 2007 but the same issue remains in 2010. Just had it.