Ambiguous virtual function

[John Chapman via Digitalmars-d-learn]

I get an "ambiguous virtual function" error when I compile this:
  interface I {
    void fun();
  }

  mixin template F() {
    void fun() {}
  }

  class C : I {
    mixin F;
    mixin F;
  }

But the error doesn't occur with this:

  class C : I {
    mixin F;
    void fun() {}
  }

Is the compiler giving the non-mixed-in function special 
treatment?