As usual, your December invoice contains your usage of December 2015. The second invoice contains the total of your 2015 payments for top-ups that haven't been fully used yet(January to December). You hadn't received an invoice for those yet, since invoicing is based on your usage.
So what do you do with these invoices? In order to close the entire financial year 2015, you need to book both invoices. You'll then notice all of your payments have been invoiced and there will be no more unpaid amounts.
As of February 2016, we'll revert to invoicing in a more simple manner. More information about this will obviously follow asap.