System 'remembers' deleted virtual servers

I am trying to migrate a domain from Server A to Server B. Server B is a brand new Virtualmin installation.

1. I created example.com on Server B by restoring from a backup from Server A. All went well, but the customer was not ready for the transfer to go live.
2. Customer is now ready to go live and so I took a fresh backup from Server A and deleted the first installation of example.com on Server B.
3. I checked that no Virtual Servers were listed on Server B and so the initial installation had indeed been deleted.
4. Then I tried to restore my new backup to Server B but the system wouldn't let the restore run, telling me that I already had a server called example.com installed on Server B.

All suggestions most welcome! I could uninstall Virtualmin and re-install it theoretically, but this is a process that I am likely to need to repeat once Server B has gone live, so while it might work for this one customer it won't work again for any future customers.