LUSTRIA: PLACES OF INTEREST If you are interested in settlements of the colonies or other races within the Lizardmen territory, you can find a lot of information here: https://forums.totalwar.com/discussion/193136/lustria-places-of-interest
So there is one Dragon Isles in Lustria and one in the Southlands. I think this was a mistake because Games Workshop went through a period (which may or may not have ended) where new writers half-assed pulling lore from previous editions when updating army books from one book to another. In a perfect world, they would keep the old lore mostly unchanged and just make necessary retcons to explain the new models they released, but we don't live in a perfect world. Official GW sources seem to have settled that the Dragon Isles is in the Southlands with official lore. I am trying to put more pins in the 8th edition Lustria map, and I don't want to Purple Cow the Lustrian Dragon Isles out of existence (Purple Cow is a verb). So what if there are TWO Dragon Isles. The Southlands Dragon Isles can be the official one and we can make the Lustria version into something else. Maybe something with like the Japanese inspired Lizardmen of @Warden 's fluff (though we need an explanation why they lost contact with the Lustrian Lizardmen when they are that geographically close, or we can make these "Dragon Isles" literal and put a bunch of dragons there. Or something else.
I think they just have the same name. There are many places that are geographically repeated in our real world, such as Santiago de Compostela in Spain, Santiago de Chile and the Santiago del Estero province of Argentina. All three are called Santiago.