What is the Compiler Doing?

I am using VC++ 6.0.  What happens when a file such as #include<iostream> is inserted into several different files all within the same project?  Obviously, it is not necessary to include a file more than once but it seems to be done quite often.   Does this hinder the performance of the executable?  What exactly is happening?  Just wondering...
guile1Asked:
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BinderCommented:
  No, it don´t hinder the performance of the executable, just
the performance of the compiler (a little bit). But there is a way
to minimize this effect. Looking at the start of iostream.h we found:

#ifndef _INC_IOSTREAM
#define _INC_IOSTREAM

....
#endif

   and of course several other lines of code. This means that
if the file wasn´t already included (if the symbol  _INC_IOSTREAM
wasn't defined yet) then the compiler includes it. If the symbol
was already defined, the compiler didn´t include it again.
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nietodCommented:
The use of #ifdef in that maner is very common.  It is necessary because C/C++ has rules against multiple definitions of the same "thing" in the same translation unit.  Without those #ifdef guards, the code would probably not compile.
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