Dual citizenship was a feature of the French colonial system.
In the French colonial system, the concept of dual citizenship existed under the idea called "assimilation." This meant that individuals living in French colonies, if they adopted French culture, language, and law, could gain some rights similar to those of French citizens. However, this was not automatic and usually applied to a select group within the colonies.
The French had two main groups in their colonies: "citoyens," who were considered full French citizens and had extensive rights, and "sujets," who were subjects of the French Empire with fewer rights. Through the process of assimilation, a subject could become a citizen, thus obtaining French citizenship while retaining their original identity, thereby leading to the idea of dual citizenship.
This approach was somewhat unique to the French system compared to other colonial systems where such a concept did not prominently exist.