>cat fun.d
module fun;
void main(){}

>cat main.d
module main;
import fun;
void main(){}

>rdmd main
Internal error: ../ztc/machobj.c 2149
//instead of gracefully giving an error that main shouldn't be defined elsewhere

This is specific to main, as the following compiles fine:

>cat fun.d
module fun;
void fun(){}

>cat main.d
module main;
import fun;
void fun(){}
void main(){}