I'm not sure why the form should be duplicating results. Unless for some reason the form is being submitted twice.
I've tested on the current release with the default form and only get one entry per submission.
What happens when you use the default form?
With the current release, duplicates are permissible, but you can limit the form to be submitted by registered users and the results do contain the wordpress user id to allow you to see who has submitted more than once.
If you test the plugin with the default form and find it is submitting more than once, please try it with all other plugins disabled. If it still happens let me know as much about you setup as possible.
If the default plugin only submit once, please tell me as much about the form you created as possible.