We had the similar issue and Tridion created a hotfix CME_2013.1.0.88281 for us which enables multivalued fields to be populated even after the first instance. It works with everything apart from keywords and categories. I would suggest not using categories and keywords and then it will work fine.
We also raised request for categories and keywords and it is right now added as a defect which will be added in the next major release. Defect # 88297