Following rP22958 (related but not dependent cleanup in rP22952, rP22969) and this Dropdown diff, we can show a warning if a C++ author calls AddSetting on the same property more than once instead of silently ignoring that.
If there is code (especially C++ code), the reader thinks the code is purposeful, but in these cases its misleading as the setting was registered already.
If the derived class calls AddSetting it implies that the derived class has the freedom to chose the type T, when it hasn't, hence misleading.
The AddSetting call should be in the class that has all the GetSetting calls to it.
So this LOGERROR will help structure (keep structured) the codebase so that AddSetting and GetSetting calls will be in the same class, thus allowing to make those setting values members,
so that the class using them will be able to access them more quickly, as laid out by Vladislav in http://irclogs.wildfiregames.com/2019-09/2019-09-20-QuakeNet-%230ad-dev.log