Someone can still throw in an #undef __YOUR_HEADER_H__ somewhere, and while that may be playing with fire, the spec says that the header file should be included again. There's so many corner cases (and ifdefs are used for so much more than guards) that you really do need to reevaluate it every time.
It really doesn't seem that complicated. When reading an include file the first time, check if everything is enclosed in a preprocessor conditional, one without else-branch. If so, remember the condition and re-evaluate it before opening the file again.
