The Create Content Wizard should just be overwriting automatically whatever previously generated content exists in your sandbox each time to access and update an XML via a Web Form. You should only get this error message if you attempt to create a brand new asset with the same name as an existing asset without first having deleted (and submitted the deletion) the it.
I have seen people delete an asset in their sandbox, which is sitting in the Modified Items list, and then try and create a new asset with the same name. If you don't submit and commit the deletion, this will not work - technically, you still have a deleted version of that asset in your sandbox.
Hope that helps.
Kevin