If you have a csv document with a list of clients, you should be able to import them, and it would detect duplicates too.
The email address is used to create a log in and it’s assigned to a contact rather than the client itself. Although with the import you are able to create both in one go.
Take a look at this topic for help with importing:
http://community.quickfile.co.uk/t/importing-clients-and-suppliers/8891