French titles, as well as Italian titles, could be shared, same happened to certain German titles, everyone is entitled to be prince or princess, same as duke or duchess but not all of them rule the area, just one. To make matters even more difficult to get, in Germany, every single child of a Count (Graff) is entitled to be either count or countess, Graf or Gräfin, the Russians did as much, again, complicating matters for outsiders. In War and Peace, Natasha is always referred as Countess while the title is being actually held by his father. I know...
During the gilded age, the rich Americans women had troubles assessing the French nobles as to rank, or who was senior within the family, Napoleon III's fall just helped them on that regard for France being yet again a republic meant those titles worth nothing anymore, sure, they were still aristocrats with castles and all but that wasn't what they were looking for. I guess this one of the main reasons, including marrying Roman catholics, as to why they were not as many marriages to French nobles, same applies to Italian, and German ones.
England, on that regard, made always things easier for outsiders, the peer is easily recognizable, same goes to the heir should there be a courtesy title, some families even invented one when there wasn't any as to make it plainly who the heir was. Same happens in Spain, there is only one peer, the duke, viscount, lord, etc, and their children are simple mr/ms, don/doña, unless the father or mother chose to relinquish the titles earlier in their favour, i.e. the Duchess of Alba did so.
I hope this helps.
Funny story, and a bit of an O/T, in France there was a class difference in between those who have old titles, what we call Nobles of the Sword (noblesse d'épée), that earned the titles via military success or just by doing some service to the monarch, or even they were there before France as an entity was created, thus tracking the origins to the old French dukedoms, and counties. An ex of mine claimed his grandfather was the vicount of Libes, now comes the funny part, there is no record whatsoever for such title, none, he tried in 2010, and earlier than that, to find some record of them in France but there wasn't any. The revolutions, sure, plural, makes it easier to invent such titles, and profit on the abroad, especially when you had 3 kings of different dinasties, even if both the Orléans and Bourbons have the same origins, and are fact both Bourbons, plust the Bonaparte, so there are a lot of nobles there. Again, tricky, and my ex was told that even if no records could be found, this didn't actually meant his family wasn't titled for the upheavals meant a huge loss of documents. In the end, it is a mistery, and a joke for his mother given sure, they were "titled" but as poor as a mouse, peasants haha. Her side of the family was the wealthy one.