It's not possible to directly include one JSON file into another. If you avoid duplication, your best bet is probably to have appsettings.json and appsettings.<env>.json. The final config is basically a sum of these two. If there are duplicates, appsettings.<env>.json has higher priority.

.NET config is not limited to .json files. It uses, among other things, environment vars. So you might try your luck wiht dontenv files