Attribute already defined when using function for it's definition

I couldn't find this documented anywhere,
although it seems like reasonable compromise in implementation.

nix-repl> {a = ({} // {}); a.b = {};}
error: attribute 'a.b' already defined at (string):1:2

       at «string»:1:18:

            1| {a = ({} // {}); a.b = {};}
             |                  ^
            2|

I find this behavior annoying, especially when using custom functions for configuration like this

{
  xdg.configFile = generateTSConfigs [  ... ];
  xdg.configFile."nvim/init.lua".source = ...;
}

Start by reproducing the expression in nix-repl and compare the behavior of the function-generated attribute with the nested xdg.configFile assignment. The issue does not name implementation files or tests, so first identify the attribute-definition and merge semantics entry points; done requires an agreed behavior and a regression check for these examples.