I tried this approach, but get;

Item has already been added. Key in dictionary: '512' Key being added: '512'

Any ideas?

OK, this is the process I use.

1. Add the url of the site you are going to put the copy on as a Portal Alias, otherwise you won't be able to access the site.
2. Create an empty database to support your second site
3. Backup your database, and restore to the empty database. Add the user that you specify in your web.config to the database with DBO permissions. - There are otherways of copying the database, but I'm old fashioned, and this is what I do.
4. Copy your complete directory structure intact to the second server.
5. Modify your web.config to point to your restored database.
6. Access your second site, and wait for application to fire up.

Job done.

Cheers
Roger

Very clear, and yet I don't understand it. The db is hosted externally, and remains the same for the "new" (read; moved) site. So why all the steps with backup and restore?