Yes, if you import a contact manually from your device's address book more than once, it will be duplicated to your Index address book.
Index 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 Index address book. (Instructions)
Questions? Submit a request.