title: Including file known to already be included
body: <p>What's the convention when including a file you know is already included by another include? </p>
  
  <p>Example: 
  Class Base is declared in "base.h" and has a few child classes.  Base has a virtual method foo(Bar&amp; bar) and therefore includes "bar.h".  From a proper convention standpoint, should we include "bar.h" in the child classes as well, given that the child classes include "base.h", which includes "bar.h"?</p>