The g++ preprocessor: Include Guards

[gutmant]

Say you have a file, a.h with an include guard.
If you include it twice and look at the preprocessed output, you see
there's no sign for the second inclusion.
However, if you include it twice - once from a relative path, and once
from an absolute one - you see that the second inclusion indeed occurs
(enters the file and leaves immediately due to the include guard).

Why does this happen(I have my speculations, but I want some
reassurance...), and is there any way to make it always act like in
the latter case?

Thanks,
 - Tali