Yes, if you add a contact from your iPhone address book more than once, it will be duplicated to your Sideline Pro address book.
Sideline does not recognize that you are duplicating a contact (name or number), so thus there is no warning message stating that you are attempting to add a contact that already exists.
Not to worry though, you can easily fix this by deleting the duplicate contact in your Pro address book.
Please visit 'How to import, create, edit, and delete Contacts' for more information on how to delete contacts from Sideline.
Questions? Submit a request.