A developer in my team keeps developing additional features to User Stories that were not required by the PO. Since they are quite useful the PO does not object them but it causes the team to never finish the committed stories. The team & PO fail to see that all those secret features mess up all planning & estimating, since they are "useful". Suggestions to add an extra story are rejected because it causes "overhead". Both developer & PO are quite new to agile development. Any suggestions?