Restore SMS database fails

I am testing the backup and recovery of my SMS site servers and when trying to restore the SMS site database I get the following error message:

ODBC SQLSTATE: 42000 Backup set holds a backup of a database other than the existing.

The scenerio I have been doing is that I will backup the Site Server using smsbkup.exe then if I try and restore with the bkupdbsite.dat that was created it works fine. BUT if I distroy the sms site then rebuild the machine and try to restore from the exact same backup as before I get the error above. It makes no sense to me. Please I need help!!!