[Greg Jones]

In addition to what Brian suggested, also look at the project's 'Mark complete when completing last action' status using the Inspect command/menu bar button. If this option is checked, then that list will be completed and hidden once the last task in the list is completed. I expect this setting, combined with the 'Create when next needed' preference, is what caused the duplicate lists.

While you are at it, you might want to look at your preference for 'Data>Projects and Action Groups>When completing the last item...' to see if it is checked or not. If it is, then by default all new projects and single-action lists will be hidden when the last task is completed as described above. For someone just getting started with OF, it might be best to turn this off for now. Once you have an established database, this setting can be enabled if needed as it will not override whatever settings you have already assigned to your lists using the inspector.