Example:

module main;

struct Foo
{
    int a;
}

extern (C) void bar(Foo);

module foo;

struct Foo
{
    int a;
}

extern (C) void bar(Foo);

$ ldc2 main.d foo.d
main.d(8): Error: Function type does not match previously declared function with the same mangled name: bar

C functions doesn't have a mangling. I suspect the mangled name that would be used for a D function for the Foo struct is used.