You can restore a site collection to the same web application it was made

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 operation

Even 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.