Where is the "virtual" keyword necessary in a complex multiple inheritance hierarchy?

I understand the basics of C++ virtual inheritance. However, I'm confused about where exactly I need to use the virtual keyword with a complex class hierarchy. For example, suppose I have the following classes:

            A
           / \
          B   C
         / \ / \
        D   E   F
         \ / \ /
          G   H
           \ /
            I

If I want to ensure that none of the classes appear more than once in any of the subclasses, which base classes need to be marked virtual? All of them? Or is it sufficient to use it only on those classes that derive directly from a class that may otherwise have multiple instances (i.e. B, C, D, E and F; and G and H (but only with the base class E, not with the base classes D and F))?