In the official lists of settlements of the second half of the 19th — early 20th century, there are so-called double names, the second name usually given in parentheses after the first one. In the article, such double names are compared with the modern system of double (simultaneously existing) names of settlements identified during the fieldwork in the second half of the 20th — early 21st century. The phenomenon of double names is considered on the example of Belozerye — a vast territory corresponding to the Belozersk Principality within the borders of the 14th century, reconstructed by A. I. Kopanev based on the study of the land tenure system. Historical data were extracted from the settlements lists of the specified territory for the Vytegorsky, Kargopolsky, and Lodeynopolsky counties of the Olonets Governorate (1905), Ustyuzhensky county of the Novgorod Governorate (1911), Cherepovets, Kirillovsky, and Belozersky counties of the Novgorod Governorate (1912). Materials for a number of volosts were compared to the settlements list of the Olonets Governorate (1873), to a map from Schubert’s Atlas reflecting the state of the area in 1826–1840 and containing double settlements names, as well as to a map from the “Novgorod Collection” of 1865–1866. The richest collections of the electronic database of the Ural Federal University Toponymic Expedition are used as a source of modern data, including materials of field expeditions to Belozerye of the 1960s–2010s: this is the territory of the west of the modern Vologda Region (Belozersky, Babaevsky, Vashkinsky, Vozhegodsky, Vytegorsky, Kirillovsky, Kaduysky, Ustyuzhensky, Chagodoshchensky, Cherepovets, Sheksninsky Districts), and south-west of the Arkhangelsk Region (Kargopolsky and Konoshsky Districts). The study examines the issues of the evolution of oikonyms over time, the relationship of various written (official, including cartographic) and oral variants, etc. The analysis of factors determining the preservation and continuity of the toponymic system is presented.