The rules governing the plural of proper nouns are complex and flexible.
The plural agreement is made:
When the proper noun refers to illustrious families:
les Bourbons (the Bourbons), les Stuarts (the Stuarts).
Proper nouns referring to other families than those remain singular:
Les Dupont sont vos voisins (the Duponts are your neighbors).
Proper nouns referring to people of the same artistic or intellectual caliber as the original bearer of the name take the plural form:
Les Picassos sont rares (people like Picasso are rare).
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