hey Simon,
When you are creating the dataconnection to submit the form to the
sharepoint library, you need to specify an autogenerated name.
On the second step of the wizard when you specify the url for the sharepoint
library there is a second text box called filename, you can put an xpath
expression in here that uses some of the fields in the form to create an
unique name. Probably the easiest way to do this is to use the concat
function to concatinate multiple fields together
e.g. concat(<field1>, <field2>, <field3>)
your other option is that you can tick the box that says "Allow overwrite if
file exists" but if you did this and didn't specify a unique name you only
ever get one form in the library and each time a user submitted that form
would be updated.

Hope this helps
Cheers
Ben