As desired, the following code fails to compile:
unittest {
    interface Foo {
        int a();
    }
}

unittest {
    Foo f;
    f.a();
}

However, the following code *does* compile:
unittest {
    interface Foo {
        int a();
    }
}

unittest {
    interface Foo {
        int b();
    }

    Foo f;
    f.a(); // we have the wrong Foo here
}

and if f.a() is changed to f.b(), it doesn't compile.