Some People say, that the border goes over the main summit, so the main summit is the highest point of Italy and France. So otherwise MBdC must be the highest summit of Italy which mustn´t be shared with an other country. In my map, the border goes around main summit and courmayeur, so that both peaks are on the french site of the border. Mont Maudit is in my map really shared. I think it is very difficult to say, because everybodys opinion is based on other secondary literature. Perhaps it is like Matterhorn, the border runs over the top, but the highest point is in Switzerland.
Maybe this question comes up to an international conflict. ;-)
In the french maps the frontier runs from the Bosses ridge (p.4740 m) across the rocks to the Brouillard ridge and then to the top of Mont Blanc de Courmayeur. Then it leads to the direction of Mont Maudit.
So Mont Blanc is the highest french peak and MB de Courmayeur the highest italian peak.
I hope that there will not develop a conflict between these contries... ;-)