I rarely mess with scriptwriting format, but have noticed that the Scriptwriting menu of script formats has every item doubled up on it. I poked around and the only place I spotted script formats stored on my hardrive is in Application Support > Scrivener > ScriptFormats, but there are no duplicates among the six xml files there (files all dated Feb 6, 2009).
Scrivener now bundles the default formats with the program itself, leaving the Application Support directory for user-created script formats. So if you have files in the App Support directory, they will appear there in addition to the ones built into Scrivener. You can delete the items in your Application Support folder to fix this.
All the best,
Keith